How do I clear my App Store credit?

You’ll either need to spend it or if it’s not enough to purchase a product or service, you’ll need to contact Apple Support to have your account set to zero. As an alternative, add a payment method and purchase an item for more than your account balance. The purchase will first be withdrawn from your account and then any remaining balance with be charged to the payment method in your account.  If you’re in the US please call  1-800-MY-APPLE or outside the US, consult the support articles below.

Contact Apple for support and service - Apple Support

You either need to purchase something that costs more than the amount you have there and use your credit card to charge the rest, or you need to contact iTunes Support and ask them to zero your balance. Use the Support link at the top or bottom of the page here.