Algo requires an API key from your Vultr account in order to create a server. The API key is generated by going to your Vultr settings at https://my.vultr.com/settings/#settingsapi, and then selecting "generate new API key" on the right side of the box labeled "API Key".
Algo can read the API key in several different ways. Algo will first look for the file containing the API key in the environment variable $VULTR_API_CONFIG if present. You can set this with the command: export VULTR_API_CONFIG=/path/to/vultr.ini. Probably the simplest way to give Algo the API key is to create a file titled .vultr.ini in your home directory by typing nano ~/.vultr.ini, then entering the following text:
[default]
key = <your api key>
where you've cut-and-pasted the API key from above into the <your api key> field (no brackets).
When Algo asks Enter the local path to your configuration INI file (https://trailofbits.github.io/algo/cloud-vultr.html): if you hit enter without typing anything, Algo will look for the file in ~/.vultr.ini by default.