Here are some stuff you can do to solve this error message:
Cannot find 'firebase' in scope.
Firstly, check if that solution works for you: https://github.com/firebase/firebase-ios-sdk/issues/6066#issuecomment-662580211
If not, check if you're importing the right library to work with firebase and swift. Take a look in my code and see if your is the same:
import FirebaseFirestore ... private var db = Firestore.firestore()