Possible reasons..
- May not upload google.services.json file to the asset
- May uploaded the same but with other than above mentioned name
- May add empty firebase blocks
- Firebase URL may not be included
- Package name mismatch
Suggestion.
- Delete the existing/uploaded google.services.json and Upload the fresh google.services.json
- Do not change the name.
- Verify in you blocks section that all the firebase components have filled.
- Add firebase url into your JSON file using textpad between project number and project ID like below,
"firebase_url": "https://(your fireDB URL)"
- Correct the package name