Why can’t I use the money on my Apple ID to buy something on an app?

The Family Organizer having a valid payment method is a requirement for anybody in the family group making a purchase while in Family Sharing. This even applies if you plan on only using your balance to pay for the item. See: Set up Family Sharing - Apple Support

Your Organizer has a problem with the Mastercard and needs to either contact the financial institution and see what the problem is, or add a different payment method.

For your Family Organizer:

Review this document about what to do "If your payment method is declined in the App Store or iTunes Store" Click here --> If your payment method is declined in the App Store or iTunes Store - Apple Support It mostly just suggests you make sure your payment method is up to date or to try a different one. Apple is just checking with the system linking to your financial institution and asking if the payment method is valid. The system is providing a negative response. Make sure the name and address are entered exactly as it appears on your financial institution bills and that the 3 digit CVV is correct. Check with your financial institution about the status of your card.

Make sure this is one of the 'Payment methods that you can use with your Apple ID'; click here --> Payment methods that you can use with your Apple ID - Apple Support A solution may be to use a method from the same country as the store you are using, but Apple no longer states either way if this is a requirement (it used to say, "Make sure that the billing address for your payment method matches the billing address for your Apple ID" which was more helpful than saying nothing the way it currently does). Also read any footnotes in the document for a payment method. Unfortunately the list is somewhat general and does not list individual brands of cards (I guess there are too many and are constantly changing).

If you still have problems, contact official Apple Support. Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

See a list of Apple phone numbers around the world, in this article --> Contact Apple for support and service - Apple Support