If you are unable to turn on "Use iCloud" within your 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 to Sync Documents" in Goodnotes.