I get this warning on a fresh installation:
Warning: The WHMCS Base URL definition is missing from your active template. Please refer to https://docs.whmcs.com/WHMCS_Base_URL_Template_Variable for more information and details of how to resolve this warning.
See screenshot with error. There is pretty good documentation about this here: https://docs.whmcs.com/WHMCS_Base_URL_Template_Variable
It's also pretty clear what to do, make sure that directive is in the `head.tpl`.
Fact is, fresh installation, happens with both Template Six and 2021. Any clues?