Park Assist not working

Hi, the Park Assist on my 718 is not working. I get Park Assist warning on the dash and am unable to access the screen on the PCM. The overhead on/off button flashes red all the time including after pressing it. I’ve checked the sensor, polished them and there is nothing ‘getting in the way’ or loose. I read about metal number plates potentially being a problem, mine are plastic. Any ideas for what to try next? Thanks