Screen time passcode reset

I followed the apple support instructions on how to reset my screen time passcode because I couldn't remember the original one and I need it in order to reset my phone before selling it on elsewhere (I have recently upgraded to a newer phone and contract).

However, after resetting the passcode it still does not work and doesn't recognise what it has been changed to. I switched the phone off and on again but with no luck so far, and continues to not be recognised.

Is there any way you can help please?