I have submitted a build of an app to TestFlight on App Store Connect. When I try to add my Gmail address, which is authorized as an Admin in Users and Access, as an Internal Tester for this app, I encounter the following error:

Here are the solutions I have tried so far:

Removed and re-added my email account in Users and Access, but the issue persists.
Updated the account’s role to Developer, but the issue persists.
I have contacted Apple Support and am awaiting a response.

The only suspicion I have is that the phone number I entered for the Account Holder is the same as the one used for the account I am trying to add as an Internal Tester. Could this be causing the issue? If not, any other solutions would be greatly appreciated.