Hi, is your issue still not resolved?
I would suggest the following steps (even if they are obvious, you still have to double-check everything):
1. Look if the google-services.json (for Android) or GoogleService-Info.plist (for iOS) files are correctly configured with your Firebase project. These files should contain the proper project ID, Sender ID, and API key. Update your client if possible.
2. If the configurations seem correct but the issue persists, regenerate your Web API Key in the Firebase Console and update it in your server-side code.
Or at least my reply will bring the attention of someone more experienced