@lamin. You need to create a testuser account in itunesconnect (it requires an email address for a fake account to be set up in iTunes - create a dummy email account in gmail is my advice). Once the account exists you can log into it on your test device (log into it through iTunes or Appstore on the device). When your test app then tries to connect to gamecenter you should us the test uswer account to log into gamecenter.
It is not a perfect system. Often it take many times for it to work and you see the user account in gamecenter. You must make sure you are logged out of any other iTunes user account on your device.
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Once the account exists you can log into it on your test device (log into it through iTunes or Appstore on the device).
When your test app then tries to connect to gamecenter you should us the test uswer account to log into gamecenter.
It is not a perfect system. Often it take many times for it to work and you see the user account in gamecenter. You must make sure you are logged out of any other iTunes user account on your device.