@trickybix13: If the issue persists following the steps above, then please restart your phone in Safe Mode, and attempt to add a contact. To enter Safe Mode, press and hold the Power button until the 'Power off' prompt appears on the screen then release > Touch and hold Power off until the 'Safe mode' prompt appears then release > To confirm, tap Safe mode.
Once in Safe Mode, try to add the relevant contact. If you're able to proceed as expected then please restart the phone as you would normally, and you should hopefully be able to add any further contacts without restriction. Let me know how you get on.