Why won’t it let me use my gift card balance to pay my subscriptions

You can have a credit card / debit card on file and an Apple Account balance.

If you have set your Apple Account balance as your default payment method, your default payment method will be used first by Apple when you need to make a payment.

If you don't have a credit card or don't want to use a credit card, you can also use your Apple Account balance as payment method.

If you have enough money in your Apple Account balance, all you have to do is -> you need to set your Apple Account balance as your default payment.

See -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

See -> What you can buy with your Apple Gift Card or Apple Account balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

and ⤵️

• You can't use your Apple Account balance to send gifts or gift cards.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are only available to you. Find out more about how purchases are billed to your Apple Account balance.
• Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

If you have followed all the steps and instructions and if you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.