Cause of error setting up Sign in with Apple for Firebase in Swift on iOS 13?

Is it possible you are using the wrong credential method? ON the documentation it looks like the one that takes the nonce is:

 let credential = OAuthProvider.credential( withProviderID: "apple.com", IDToken: appleIdToken, rawNonce: rawNonce )

but the one you're using here takes an access token, not sure if that helps.

I ran into the same error.

Solution: Just run

pod update

Explanation:

The problem is as @ethanrj says. The documentation says to do

let credential = OAuthProvider.credential( 
    withProviderID: "apple.com", IDToken: appleIdToken, rawNonce: rawNonce )

but this gives an error and Xcode will suggest the following (rawNonce -> accessToken):

let credential = OAuthProvider.credential( 
    withProviderID: "apple.com", IDToken: appleIdToken, accessToken: rawNonce )

This is because pod install will install FirebaseAuth 6.12 by default, when you really need 6.13 since the new function signature is only available there. You can view the source here.