I am a test engineer. When I subscribe with a sandbox account in a test project, I find that the prompt "Not allowed to create an iTunes account" appears, but my account was successfully added in the sandbox. I tried two different email suffixes @gmail.com and @tester.com, and both showed this prompt. The developers in the team were also unable to solve this problem. Does anyone know how to deal with this situation?
Just to confirm, you installed the app via Xcode adhoc distribution? If so, you would use the Sandbox Apple Account created in App Store Connect - that is signed in on device under Settings app > Developer > scroll to bottom. See Sandbox testing article for more info.
If still having issues please file a feedback assistant ticket and post the FB# here and we can followup.
