Try below steps:
Enable Keychain Sharing for iOS: Go to Signing & Capabilities in Xcode > Add "Keychain Sharing" capability. This is required for Firebase Authentication on iOS starting with Firebase iOS SDK 9.6.0.
Make sure your GoogleService-Info.plist is correctly placed in the iOS project and matches your Firebase project settings.
Make sure firebase_auth and firebase_core are up-to-date. Run flutter pub upgrade.
Run flutter clean, then flutter run to rebuild the app with correct configurations.