Cannot change or turn off passcode for screen time on iPhone

Click here: If you forgot your Screen Time Passcode

If the above doesn't work, when you first create a screen time passcode it prompts for an optional Apple ID to recover the passcode if you forget it. This doesn’t have to be the same Apple ID that the phone is using; it might be a parent’s Apple ID, for example. This is the Apple ID you need to reset the Screen Time passcode. If you didn’t set one up (it IS optional) or don’t know that Apple ID then you can’t recover the screen time passcode; the only option is to reset the phone to factory settings and set up the phone as New→Restore your iOS device to factory settings - Apple Support