I have $15 gift card on my account. I want to purchase something in one of my apps for $9.99 but it's still trying to charge my credit card. Why??

Has the organiser does as the message instructs them to do ? That message means that iTunes wasn't able to collect the money for a member's purchase due to a problem with the organiser's payment method, until that is corrected and the member's purchase paid for no other purchases can be done by the family members.

How can you be certain of this?

As it says, before you can make additional purchases with any (gift or credit) card, you must rectify whatever problem exists with a family member's past purchase(s).