When your tow ball is fitted they it would be normal to adjust the electrics so the reverse parking sensors disable when the tow ball is in use (i.e. when there is something plugged into the lighting socket). This would prevent the parking sensors making a noise when a tow bar bike carrier (or trailer) in being used. Do check this to be sure your tow bar has been installed in this way. If not, you'll have to manually disable the parking sensors while you are towing, or they will pick up the tow bar bike rack.
Any reversing cameras should still work, even when you have something on the tow ball. You'll obviously get a restricted view from the camera if you are using a tow bar bike carrier (and the view may be completely obscured if you are carrying a tow bar cargo carrier).