Please open your iPad Settings (not GoodNotes Settings), tap Apple ID > iCloud, and switch on both "iCloud Drive" and "GoodNotes"? Then you should be able to enable "Use iCloud" in GoodNotes:
I can't turn on "Use iCloud" in GoodNotes.
Please open your iPad Settings (not GoodNotes Settings), tap Apple ID > iCloud, and switch on both "iCloud Drive" and "GoodNotes"? Then you should be able to enable "Use iCloud" in GoodNotes: