Unfortunately, the "Attachment not found" error on export indicates that the document is corrupted. This could happen if the app was deleted unexpectedly while doing some ongoing file operations like syncing to iCloud.
If you have GoodNotes synced across another device as well, please check if the corresponding document has this issue on that device?
- If no, please turn off "Use iCloud" on the device with this issue, force-quit the app and relaunch it, then turn "Use iCloud" back on. See if that helps resolve the error.
- If that other device also has this issue with that document (or you don't have another device), what you can try is to restore from a device backup. If you open the Settings app > Apple ID > iCloud > iCloud Backup and see that "Last successful backup" shows a time earlier than this incident, please restore from that iCloud backup as soon as you can!
Failing the above, you will need to export the document as a PDF, re-import the file and use it going forward, even though the existing annotations aren't editable, unfortunately.