Why can't I use an Apple gift card for in-game purchases while being a member of Family Sharing?

Hi TadnessMadness,

Welcome to the Apple Support Communities! I'm sure we can help you out with your question. Generally, if you are a member of a Family Sharing group, purchases you make will be billed against any available gift or store credit on your account before they are billed to the Family organizer, as noted here:

Making purchases

After you set up your family, any time a family member initiates a new purchase it will be billed directly to your account, unless that family member has gift or store credit. First, their store credit will be used to pay the partial or total bill. The remainder will bill to the family organizer's card. As the family organizer, any receipts generated by the transaction will be sent to you. Learn more about how iTunes Store purchases are billed.

Family purchases and payments - Apple Support

If you have an iTunes Store Gift Card, you would want to make sure that it has been redeemed on your account so that any credits will be available for your purchases:

Redeem iTunes Gift Cards and codes - Apple Support

Note: For in-app purchases, make sure that you are using an iTunes Store Gift Card and not an Apple Store Gift Card, as noted here:

What type of gift card do I have? - Apple Support

Cheers