Setting sensitivity label to classic site using CSOM

[Vivek00001]

I am trying to set sensitivity label to existing classic team site using the following CSOM code.

ClientContext.Site.SensitivityLabelId = sensitivityLabelId;
ClientContext.ExecuteQueryRetry();

I am getting "Attempted to perform an unauthorized operation" error
I am using App-only token and App has Sites.FullControl.All application permission (SharePoint).

Could you please help in identifying issue.

[LeHailender]

Any updates here?