The only success I've had testing Apple Sign In with Xcode 12 and iOS 14 is to use a real device with a sandbox environment. I tested with an iPhone 11 Pro with iOS 14 installed.
ukaton
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This is also a significant blocker in our development and I have tested Apple Sign In. There's no way to see any errors since the activity indicator seems to hang and the console doesn't indicate any Authentication/Authorization errors.
About Xcode
Version 12.0 (12A7209)
About Simulator
iPhone 8+ (iOS 14) [Works on iOS 13]
Version 12.0 (940.16)
SimulatorKit 597.13
CoreSimulator 732.17