If you are unable to turn on "Use iCloud" within your GoodNotes 5 app, then it's possible that you may need to enable the feature directly through your device settings.
To to this, open your iPad Settings (not GoodNotes Settings), tap Apple ID > iCloud, and switch on both "iCloud Drive" and "GoodNotes". From there, you should then be able to enable "Use iCloud" in GoodNotes:
Notes:
- To turn on "Use iCloud" in GoodNotes for macOS, please look for these switches in System Preferences > Apple ID and "Options" next to "iCloud Drive":
- If you still can't turn on "Use iCloud" despite having taken the steps above, try restarting your device/machine.