Firebase Permission Denied

For others struggling with this problem, the proper solution is to change the false in

service cloud.firestore { 
    match /databases/{database}/documents {
        match /{document=**} { 
            allow read, write: if false; 
        } 
    }
}

To true while testing. Don't forget to add additional security when going live with your project!

I was able to solve the problem by switching from Cloud Firestore to Realtime Database, and then changing the rule. (After I've changed this, there was no more error showing!)