From the Firestore Database, Click on the Indexes tab and click on the Add Index. You have to create one index for the database.
Enter questions as Collection ID, quiz_id into the first field and set Ascending, Enter created_at in the second field and set Ascending, Enter __name__ in the third field and set Ascending and then select Collection as query scope and save it.
This may take 3-5 minutes to activate. After completing this, your index page should look like this: