Have you changed the wheels (and tyres) sizes so they’re significantly different from stock? Have you now an overall diameter difference front to back?
If either of the above then the wheel speed sensors can give information the control unit isn’t happy with and so won’t allow cruise to work.
If wheels and tyres all good and stock equivalent then I’d check that all the wheel speed sensors are working correctly. If you have an OBD port and a Mercedes specific scanner you can read wheel speeds.