Apple ID

Se this again -> When redeeming a gift card, (1) the amount will be credited to your Apple Account balance and (2) then you need to set your Apple Account balance as your default payment.

(1) See -> Check your Apple Account balance

Check your Apple Account balance – Apple Support (UK)

(2) See -> Add a payment method to your Apple ID

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

If you already use your Apple Account balance, the Apple Gift Card is automatically added to your Apple Account balance.

(you have done this) -> How to redeem your Apple Gift Card or App Store & iTunes gift card

How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)

When redeeming a gift card, (1) the amount will be credited to your Apple Account balance and (2) then you need to set your Apple Account balance as your default payment.

(1) See -> Check your Apple Account balance

Check your Apple Account balance – Apple Support (UK)

(2) See -> Add a payment method to your Apple ID

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

If you update your payment method or add funds to your Apple Account balance, It would take about 1 to 3 business days in order to process.

Perhaps this is where the delay lies, check again after a few days.

If you're still unable to update your payment method, Contact Apple Support.

See -> Check your Apple Account balance -> Check your Apple Account balance – Apple Support (UK)

Click here for more support and assistance.