To enable classes and or locations in Mobius, you first have to enable Catagories/Locations withing QuickBooks Online Settings.
You need to go to your company settings in QuickBooks Online to enable classes in QuickBooks Online before Gozynta Mobius will let you save this setting. Choose "Track Classes" and select under Assign Classes: "One to each row in transaction." We also recommend to click "Warn me when a transaction isn't assigned a class." Once this is saved you can click "Classes" in Gozynta Mobius and it will save.