I had the same problem and did this to fix it.
-Make sure the google-services.json is in the assets folder, and DON’T change the name of the file. Also make sure it doesn’t have a number, like google-services(1).json.
-The package name of your app, and the package name you provide to firebase to download “google-services.json” MUST be the same.
-I migrated my app from appybuilder to kodular, so you must export your keystore from the previous platform, and overwrite the keystore to the new kodular project (you can find the option in the top right corner in “Settings”).
-Then, in the “Advance” setting in the Screen1 design settings, I put the package name (Example of my migrated package name: com.appybuilder.user.projectname).
Hope this can save someone the troubles.