Can’t turn on Purchase Sharing on Family Sharing

I am the organizer and have my nephew in Family Sharing. I want to turn on Purchase Sharing and have a valid credit card (where all my apple suscriptions and purchases are charged) set up as payment option but every time I want to turn on Purchase Sharing it keeps asking me to set up a new payment option, but it never allows me to select any of the options as the Purchase Sharing payment method.

Can you help? (I am in México and so is my nephew, we already share Icloud but he can’t even download free apps unless there is a payment method set up and i want to share mine)