We once set up some generic accounts for a beta test program, and Google wanted us to add the users to a group of theirs, external to us. We had enabled all the appropriate services, but kept getting error messages when trying to get the account to join the external group.
In our case the issue was that our beta test accounts were in an OU that was not explicitly marked as having all users over 18.
You may wish to check Google Admin>Account>Account settings>Age based access settings and verify that the OU for your user is marked as "All users in this group or org unit are 18 or older" (if that is true).