Can’t change my screen time passcode

If the related article above does not help then yopu will have to DFU the phone and start from scratch

You should back up first for safety but remember that restoring that backup will restore the passcode.

First export all your data, photos etc to cloud services or your computer. Your emails, contacts, calendars, Notes, remionders are safe and in the cloud already, but apps like Whatsapp will offer icloud backups fro within their own settings,

Some message history like texts will disappear forever.

Then Recovery mode or DFU

 If you can't update or restore your iPhone or iPod touch – Apple Support (UK). (Ignore the title)