For information about paying with your Apple Account balance, see the following from: Payment, Financing, Refunds and VAT - Shopping Help - Apple
If that doesn't help, you may want to contact Apple Sales Support at 1-800-MY-APPLE
Apple Account Balance
Redeem Apple Gift Cards or add money directly into your Apple Account Balance anytime. Then use the balance to pay for Apple products, accessories, apps, games, music, movies, TV shows, iCloud, and more. You can check your Apple Account Balance online.
To use your Apple Account Balance to buy products on apple.com or in the Apple Store app, make sure you are signed in with your Apple ID. Then simply choose an amount to apply to your purchase during the payment step in checkout. You can use the funds from your Apple Account Balance in combination with Apple Pay or one credit card and up to eight Apple Gift Cards. See Payment for details on using more than one payment method.
You can redeem Apple Gift Cards and App Store & iTunes Gift Cards into your Apple Account Balance on iTunes, Apple Music, Apple TV, or the App Store. Visit any of these Apple apps or sites and tap your Apple ID account avatar or choose Account from the main menu. Select “Redeem Gift Card or Code” or “Redeem,” then scan or enter the gift card PIN. The funds will appear in your Apple Account Balance to use for shopping, listening, watching, playing, working, and more.
Please note that your Apple ID Balance is now your Apple Account Balance.