Hello Applespawn,
Does that notification appear when you try to make a purchase, or do something specific on your device, or does it seem to appear randomly? Is purchase sharing turned off? We ask because, if Purchase Sharing is turned off, everyone must use their own payment method.
You can find more information about that in this link, which explains options, and may be helpful: How to share purchases with your family
Which payment method is charged for purchases that family members make?
When Purchase Sharing is turned on, everyone's purchases bill to the family organizer's payment method.* If there's an issue with the payment method, the family organizer can update their payment method or add a new one.
If you're the family organizer and you don't want to share a payment method with your family, turn off purchase sharing. When purchase sharing is turned off, you can continue to share subscriptions like Apple TV+ and Apple Arcade. When Purchase Sharing is turned off, everyone must use their own payment method.
* Learn how purchases are billed if a family member has Apple Account balance.
Thanks!