Unable to change Screentime passcode on iPhone

Your Mom will need to reset the Screen Time Passcode. It makes not difference if there are restrictions on your device, the Passcode has been set up and that is why you cannot change it without providing the Passcode. You Mom will need to follow the steps here for resetting the Screen Time Passcode when using Family Sharing. :

If you forgot your Screen Time passcode - Apple Support

• What happens when you try?
• Do you know the current Screen Time Passcode?
• Is your phone part of a Family Sharing plan?
• Is your phone being managed, such as for work or school?