Contact Support!
When a question regarding some Service (in this case, Google) goes unanswered for a long time, it is most likely a problem on the Service's side. As of March 2023 this question was still not answered, when we ran into literally the same issue, just to add some more details:
- Google Workspace forced us to define a domain during registration process;
- The username of the newly created account was set to "mailbox in that domain", our original mailbox was listed as a fallback;
- The newly created account has not been granted access to the Google Cloud Console or any other services other than the Workspace admin panel;
- The original Google Account didn't get the option to switch its existing apps to Internal mode with the same "not a Google Workspace user" tip.
They say that Google doesn't put much effort into supporting specific customers in their specific cases, but not when it comes to Workspace users. After a short dialogue and describing the problem, the support agent "pushed some magic buttons" on their side, then offered to sign in with those newly created credentials to the Google Cloud Console from incognito mode. Since the login was successful, we were able to register a new application that now supports Internal mode.
I still believe this is an issue on the Service side that will be resolved someday. Until then, I hope one finds this experience useful.
