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 system settings.
To do this, go to your iPhone/iPad/Mac Settings > Apple ID > iCloud > iCloud Drive, switch on "Sync this iPhone/iPad/Mac", and then also "GoodNotes" in "Apps Syncing to iCloud Drive":
From there, you should then be able to enable "Use iCloud" in GoodNotes.