Firestore PERMISSION_DENIED

Inside projectFolder/android/app/src/main/AndroidManifest.xml add the following:

 <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

According to this post, there is an error with your app accessing the internet. Please check your network connection:

Check that you have the proper internet permissions in the androidManifest file (suggested by Abhishek Dutt).

If you are using an emulator, check if the emulator can access the internet using this post.

1st edition:

Can you please try this example code from this documentation for authentication:

  service cloud.firestore {
      match /databases/{database}/documents {
        match /messages/{document=**} {
          allow read, update, delete: if request.auth.uid == resource.data.uid;
          allow create: if request.auth.uid != null;
        }
      }
    }

This code must work only for reading and not writing:

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