See the solution when running on real device: https://developer.apple.com/forums/thread/651533?page=13

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