I've been experiencing the same issue. I've created multiple sandbox testers. However, when performing a purchase, it's using the apple id instead of the sandbox login. If I log out from the "iTunes & App Store" with my apple id and use the sandbox tester email, I get the "iTunes account creation not allowed" error.
I would really like to know what it is we need to make sure our sandbox tester accounts are being used to test purchasing via TestFlight builds.
EpicQA
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