Family must have valid payment error
Family must have valid payment method error pops up when my daughter tries to download form iCloud. It shows my payment method under her user name in family sharing. Why isn't it recognizing it?
Try going to Settings>iTunes & App Store, tap the ID, choose View Apple ID, then tap Payment Information and confirm that everything looks correct. Sometimes just checking the payment information will resolve this. In other cases you may be prompted that your current password isn't secure and needs to be changed.
Then test Family sharing purchasing again.