If you see an error message that states "Failed to open document", please try the following:
Please open the Settings app > iCloud > Storage > Manage Storage > GoodNotes (it may take a while to load when you tap GoodNotes). Are the documents listed there? If they are, please try reinstalling GoodNotes. If it says some documents are still syncing when you delete GoodNotes, please wait until all documents are synced with iCloud.