Error “Your Apple ID is not authorised to make this purchase”

I have been trying to upgrade to Snapchat Plus but every time i press subscribe it says “your apple id is not authorised to make this purchase”. i have tried everything to fix it, eg. update the payment method on the app store, log out of my apple id on the app store and log back in etc. but it still won’t work, what am i doing wrong? i’m part of family sharing which my mum is the organiser and i have looked on her account but there’s nothing that says my apple id cannot make purchases so i don’t get it?

i have included a picture for reference to what comes up when i press subscribe.

[Re-Titled by Moderator]