core.js:6456 ERROR FirebaseError: Missing or insufficient permissions. code example

Example: Uncaught (in promise) FirebaseError: Missing or insufficient permissions.

When you look at your Firestore rules, they should match:

service cloud.firestore {
  match /databases/{database}/documents {
    match /{document=**} {
      allow read;
      allow write: if false;
    }
  }
}
If you do not allow read, you may see this issue in the client.