pub get failed: Because flutter_app depends on firebase_firestore ^0.12.7+1 which doesn't match any versions, version solving failed

firebase_firestore package is deprecated and the last version it has is 0.0.3+1... so it cannot find version ^0.12.7+1

You will need to cloud_firestore instead:

https://pub.dev/packages/cloud_firestore

Try following the latest Firebase codelab from the Flutter team.

https://flutter.dev/docs/development/data-and-backend/firebase