I can only buy Apple Music or Apple TV with the credit added by Gift Card on my account

I redeemed a gift card into my AppStore

When redeeming a gift card, the amount will be credited to your Apple Account balance and set your Apple ID balance as your default payment.

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 also -> 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)

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• 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 still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service 

Here is --> How to redeem your Apple Gift Card or App Store & iTunes ...