How to fix a "fails to compile a custom C# assembly"

[Ogawa-Kazuma]

I am following the Installation page and have downloaded Unity 20213.44f with WebGL Build Support, Starter Template. After opening a project from the Starter template, it prompts for an update to the SDK. After updating, I trial and error an empty scene, letting the SDK autofix the issues and tried to test the scene. It fails "compilation error fur custom C# assemblies" and gives out two error messages

Error 1
Exception: Failed to compile custom C# scripts
SpatialSys.UnitySDK.Editor.BuildUtility+<>c.b__13_1 () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Editor/Scripts/Utilities/BuildUtility.cs:255)
RSG.Promise+<>c__DisplayClass54_0.b__0 () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:900)
RSG.Promise.InvokeResolveHandler (System.Action callback, RSG.IRejectable rejectable) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:461)
UnityEngine.Debug:LogException(Exception)
SpatialSys.UnitySDK.Editor.<>c:b__13_11(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Editor/Scripts/Utilities/BuildUtility.cs:349)
RSG.<>c__DisplayClass45_0:b__1(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:677)
RSG.Promise:InvokeRejectHandler(Action1, IRejectable, Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:443) RSG.Promise:InvokeRejectHandlers(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:507) RSG.Promise:Reject(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:563) RSG.Promise:InvokeRejectHandler(Action1, IRejectable, Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:443)
RSG.Promise:InvokeRejectHandlers(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:507)
RSG.Promise:Reject(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:563)
RSG.Promise1:InvokeHandler(Action1, IRejectable, Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:359)
RSG.Promise1:InvokeRejectHandlers(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:388) RSG.Promise1:Reject(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:445)
RSG.<>c__DisplayClass52_01:<Then>b__1(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:785) RSG.Promise:InvokeRejectHandler(Action1, IRejectable, Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:443)
RSG.Promise:InvokeRejectHandlers(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:507)
RSG.Promise:Reject(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:563)
RSG.Promise:InvokeRejectHandler(Action1, IRejectable, Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:443) RSG.Promise:InvokeRejectHandlers(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:507) RSG.Promise:Reject(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:563) RSG.Promise:InvokeRejectHandler(Action1, IRejectable, Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:443)
RSG.Promise:InvokeRejectHandlers(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:507)
RSG.Promise:Reject(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:563)
RSG.Promise:InvokeRejectHandler(Action1, IRejectable, Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:443) RSG.Promise:InvokeRejectHandlers(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:507) RSG.Promise:Reject(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:563) RSG.Promise:InvokeRejectHandler(Action1, IRejectable, Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:443)
RSG.Promise:InvokeRejectHandlers(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:507)
RSG.Promise:Reject(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:563)
RSG.Promise1:InvokeHandler(Action1, IRejectable, Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:359)
RSG.Promise1:InvokeRejectHandlers(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:388) RSG.Promise1:Reject(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:445)
RSG.<>c__DisplayClass52_01:<Then>b__1(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:785) RSG.Promise:InvokeRejectHandler(Action1, IRejectable, Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:443)
RSG.Promise:InvokeRejectHandlers(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:507)
RSG.Promise:Reject(Exception) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:563)
RSG.Promise:InvokeResolveHandler(Action, IRejectable) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:465)
RSG.Promise:InvokeResolveHandlers() (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:521)
RSG.Promise:Resolve() (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:588)
RSG.<>c__DisplayClass44_0:b__3() (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:758)
RSG.<>c__DisplayClass54_0:b__0() (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:900)
RSG.Promise:InvokeResolveHandler(Action, IRejectable) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:461)
RSG.Promise:InvokeResolveHandlers() (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:521)
RSG.Promise:Resolve() (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:588)
RSG.<>c__DisplayClass45_0:b__0() (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:671)
RSG.Promise:InvokeResolveHandler(Action, IRejectable) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:461)
RSG.Promise:InvokeResolveHandlers() (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:521)
RSG.Promise:Resolve() (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:588)
RSG.<>c__DisplayClass45_0:b__0(GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:819)
RSG.Promise1:InvokeHandler(Action1, IRejectable, GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:359)
RSG.Promise1:InvokeResolveHandlers(GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:402) RSG.Promise1:Resolve(GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:470)
Proyecto26.RestClient:Promisify(Promise1, RequestException, GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/EditorRestClient/RestClientPromise.cs:426) Proyecto26.RestClient:Promisify(Promise1, RequestException, ResponseHelper, GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/EditorRestClient/RestClientPromise.cs:442)
Proyecto26.<>c__DisplayClass6_0`1:b__0(RequestException, ResponseHelper) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/EditorRestClient/Helpers/HttpBase.cs:142)
Proyecto26.d__1:MoveNext() (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/EditorRestClient/Helpers/HttpBase.cs:57)
Unity.EditorCoroutines.Editor.YieldProcessor:MoveNext(IEnumerator) (at Library/PackageCache/com.unity.editorcoroutines@1.0.0/Editor/EditorCoroutine.cs:80)
Unity.EditorCoroutines.Editor.EditorCoroutine:ProcessIEnumeratorRecursive(IEnumerator) (at Library/PackageCache/com.unity.editorcoroutines@1.0.0/Editor/EditorCoroutine.cs:134)
Unity.EditorCoroutines.Editor.EditorCoroutine:MoveNext() (at Library/PackageCache/com.unity.editorcoroutines@1.0.0/Editor/EditorCoroutine.cs:115)
UnityEditor.EditorApplication:Internal_CallUpdateFunctions()

Error 2
Failed to compile C# assembly; Output dll not found at Temp/CSScriptingCompiledDlls\WebGL\NewAssembly.dll
{
"name": "NewAssembly",
"references": [
"SpatialSys.UnitySDK",
"Unity.TextMeshPro"
],
"includePlatforms": [],
"excludePlatforms": [],
"allowUnsafeCode": false,
"overrideReferences": false,
"precompiledReferences": [],
"autoReferenced": true,
"defineConstraints": [],
"versionDefines": [],
"noEngineReferences": false
}
UnityEngine.Debug:LogError (object)
SpatialSys.UnitySDK.Editor.CSScriptingEditorUtility:CompileAssembly (UnityEditorInternal.AssemblyDefinitionAsset,string,bool,bool) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Editor/Scripts/CSScripting/CSScriptingEditorUtility.cs:165)
SpatialSys.UnitySDK.Editor.BuildUtility:CompileAssemblyIfExists (bool) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Editor/Scripts/Utilities/BuildUtility.cs:590)
SpatialSys.UnitySDK.Editor.BuildUtility/<>c:b__13_1 () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Editor/Scripts/Utilities/BuildUtility.cs:253)
RSG.Promise/<>c__DisplayClass54_0:b__0 () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:900)
RSG.Promise:InvokeResolveHandler (System.Action,RSG.IRejectable) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:461)
RSG.Promise:InvokeResolveHandlers () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:521)
RSG.Promise:Resolve () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:588)
RSG.Promise1/<>c__DisplayClass44_0<bool>:<Then>b__3 () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:758) RSG.Promise/<>c__DisplayClass54_0:<Then>b__0 () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:900) RSG.Promise:InvokeResolveHandler (System.Action,RSG.IRejectable) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:461) RSG.Promise:InvokeResolveHandlers () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:521) RSG.Promise:Resolve () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:588) RSG.Promise/<>c__DisplayClass45_0:<Catch>b__0 () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:671) RSG.Promise:InvokeResolveHandler (System.Action,RSG.IRejectable) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:461) RSG.Promise:InvokeResolveHandlers () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:521) RSG.Promise:Resolve () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise_NonGeneric.cs:588) RSG.Promise1/<>c__DisplayClass45_0<SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse>:b__0 (SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:819)
RSG.Promise1<SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse>:InvokeHandler<SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse> (System.Action1<SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse>,RSG.IRejectable,SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:359)
RSG.Promise1<SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse>:InvokeResolveHandlers (SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:402) RSG.Promise1<SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse>:Resolve (SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/CSharpPromise/Promise.cs:470)
Proyecto26.RestClient:Promisify<SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse> (RSG.Promise1<SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse>,Proyecto26.RequestException,SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/EditorRestClient/RestClientPromise.cs:426) Proyecto26.RestClient:Promisify<SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse> (RSG.Promise1<SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse>,Proyecto26.RequestException,Proyecto26.ResponseHelper,SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/EditorRestClient/RestClientPromise.cs:442)
Proyecto26.HttpBase/<>c__DisplayClass6_0`1<SpatialSys.UnitySDK.Editor.SpatialAPI/GetFeatureFlagsResponse>:b__0 (Proyecto26.RequestException,Proyecto26.ResponseHelper) (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/EditorRestClient/Helpers/HttpBase.cs:142)
Proyecto26.HttpBase/d__1:MoveNext () (at Library/PackageCache/io.spatial.unitysdk@1.67.0/Packages/EditorRestClient/Helpers/HttpBase.cs:57)
Unity.EditorCoroutines.Editor.EditorCoroutine/YieldProcessor:MoveNext (System.Collections.IEnumerator) (at Library/PackageCache/com.unity.editorcoroutines@1.0.0/Editor/EditorCoroutine.cs:80)
Unity.EditorCoroutines.Editor.EditorCoroutine:ProcessIEnumeratorRecursive (System.Collections.IEnumerator) (at Library/PackageCache/com.unity.editorcoroutines@1.0.0/Editor/EditorCoroutine.cs:134)
Unity.EditorCoroutines.Editor.EditorCoroutine:MoveNext () (at Library/PackageCache/com.unity.editorcoroutines@1.0.0/Editor/EditorCoroutine.cs:115)
UnityEditor.EditorApplication:Internal_CallUpdateFunctions ()

I don't know where and what went wrong, opening the project and creating a barebones scene to test it takes half an hour.