How do I select a payment method for Family Sharing in the App Store?

I’m trying to set up purchase sharing as a family organiser. When prompted to add a payment method I click add payment method but it just routes me to the App Store “manage payment methods” where I can see my payment methods (and add new ones or delete existing ones) but there isn’t an option to select either of them (apple pay or my debit card) as the payment method for family sharing. Unclear what I’m meant to do here? Without this I can’t set up family sharing but there seems to be no way to proceed. I’m not getting an error or being told I can do something there’s just no option to select a payment method.

[Re-Titled by Moderator]