This guide applies to the following error seen when trying to log into Supermetrics data sources - it's more common with Data Studio, but can also appear for the other products:
This action is not supported when you are signed in to multiple accounts in the same domain or on multiple consumer accounts. Log out and try again
This error happens to users that are logged into Google services with multiple accounts. It's functionally a variation on the known account management bug that has issues with multi-user logons (see bug tracker: https://issuetracker.google.com/issues/69270374).
To clear the error, log out all users from Google in your browser except for the one account you need to work the product/use the license (for example, for Google Sheets, log out all users except the one attached to the Supermetrics license). Then try to log in to the data source again.
Alternatively, if logging out all users is a hassle, you can also do this via Incognito/private browsing. Make sure that you log in with only ONE Google account (else the issue continues there too). See also Authentication-, HTTP- or Script errors.