I am also experiencing this issue.
Using Xcode 12.0, I am seeing the same "double prompt" for password, which is then followed by this error:
Error Domain=SKErrorDomain Code=0 "Cannot connect to iTunes Store" UserInfo={NSLocalizedDescription=Cannot connect to iTunes Store}
This is happening on devices running iOS 11.x < iOS 13.x.
As others have stated, this does not occur on iOS 14 devices. I tried purchasing both consumable IAPs, and Renewable Subscriptions. Both have the same result. Test Flight builds that perform purchases using non-sandbox Apple IDs seem to work without any issue on any OS.
mattfromfountain valley
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