Is the folder with the documents on your drive or a team drive? And did you create the folder or did Google Classroom create the folder?
Try moving the folder to the top of your drive, not in any subfolders.
As a test, can you create a new folder on your drive with a test document and see if you can get docappender to work on that? ( Sometimes in the past, older folders or folders not created by the owner can have this permission issue)
Also when you open docAppender can you select 'Get User Key' and paste that key on this thread, I'm not sure if there would be any backend errors associated with this but no harm in checking it out.
Thanks!