Looks to me like whatever Google Analytics account they linked, it is not an Google Analytics Administrator account for the Google Account that contains the website property. You can check the users and their allowed roles by going into the Google Analytics account, then going toAsking this on behalf of a customer When attempting to select the GA4 property after connecting the GA account, this error pops up.
Admin -> Account Access Management. If the Google Account you linked within the
Xenforo -> Options -> Analyticsarea has a role of Analyst, Viewer or None, that account will not be able to make configuration changes via the API.
You could probably get away with having an Editor role, however I've not tried it. If that doesn't work, you may need to make them an Administrator role for the Google Analytics Account.
The Google "user" that is making the underlying API calls will need access to perform administrator-level functions in order to get measurement protocol secrets for the Analytics property (what that call is trying to do).