getting Domain=com.firebase.functions Code=13 when FCMTokenProvider.getToken function to fetch Auth Token for google Cloud Service in ios swift

[I'm follow the https://cloud.google.com/solutions/mobile/token-service-oauth reference for configuration to get Auth token to use in Google Could Service but it's giving Error Domain=com.firebase.functions Code=13 "INTERNAL", Can anyone suggest me why this error coming

func messaging(_ messaging: Messaging, didReceiveRegistrationToken fcmToken: String) {
    InstanceID.instanceID().instanceID { (result, error) in
        if let error = error {
            print("Error fetching remote instance ID: \(error)")
        } else if let result = result {
            print("Remote instance ID token: \(result.token)")
            FCMTokenProvider.getToken(deviceID: result.token) { (status, string, error) in
                print(status) // false
                print(string) // nil
                print(error) //Error Domain=com.firebase.functions Code=13 "INTERNAL" UserInfo={NSLocalizedDescription=INTERNAL}
            }


        }
    }
}

]1