See the solution when running on real device: https://developer.apple.com/forums/thread/651533?page=13
ArthurYidi
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I was having the same issue, the sample code is missing a critical step (on a real device).
You must add a key with 'Sign in with Apple' enabled in your developer account:
Sign-in with your Developer Account
Go to 'Certificates, Identifies & Profiles' > 'Keys'
Create a new key (+)
Fill in 'Key Name'
Enable 'Sign in with Apple'
Click 'Configure'
Assign the primary App ID
I recommend using a standard prefix for the App bundle ID (ex. com.companyname.) in the sample code and your apps.
For more details, make sure to follow step 6:
https://medium.com/developerinsider/ios-13-how-to-integrate-sign-in-with-apple-in-your-application-c21f579aca7