Move the allow override option out of the working rota tab into a different tab to allow setting override on profile's that don't have a working rota.
It would make most sense in the Entitlement Settings tab.
Workaround for other users:
If you need to enable override on a profile with no working rota, you can enable the working rota temporarily to check the Allow Override box.
Go to Org Admin > Leave Admin > Leave Profiles > Select a Profile > Check "Automatically Calculate Working Hours?" > Tick "Allow Override" on the Working Rota tab > Save.
Then uncheck and put in the correct amount of hours.