I had the same problem with In-App Purchase on iOS 14 and sandbox environment:
Error Domain=SKErrorDomain Code=0 "An unknown error occurred" UserInfo={NSLocalizedDescription=An unknown error occurred, NSUnderlyingError=0x28385e520 {Error Domain=ASDErrorDomain Code=500 "Unhandled exception" UserInfo={NSUnderlyingError=0x28385c510 {Error Domain=AMSErrorDomain Code=100 "Authentication Failed" UserInfo={NSLocalizedFailureReason=The verify credentials call failed., NSLocalizedDescription=Authentication Failed, NSUnderlyingError=0x28385c3c0 {Error Domain=AMSErrorDomain Code=0 "Authentication Failed" UserInfo=0x282361940 (not displayed)}}}, NSLocalizedFailureReason=An unknown error occurred, NSLocalizedDescription=Unhandled exception}}}
Workaround:
Use Apple account with disabled two factor authentication.
P.S. If you cannot sign out because 'Sign out is not available due to restrictions' first turn off 'Screen Time'
pjas
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution
Last seen
User for
Post
Replies
Boosts
Views
Activity
My app was rejected when review. Please help me if you known how to handle it.
I test in TestFlight App with sandbox, it is working well. But failed when reviewing by Apple with error : "An unknown error occurred" Sandbox server is not working sometimes, just write to Apple's Testers to test it again or send a new build (only with increased build number).