You need to grant file access permission to the app so that the notes file is visible for you to back up:
When you first launch the app, select "Allow" to grant file access permission to the app.
Denying the permission still give us ability to use notes features with Plus version, but the note file is not visible for you to do backup.
What if you accidentally denied the permission?
if you change your mind
Go to settings for "App permissions"
Change the permission
Select "Allow access to media only"