I redeemed a gift card yet I cannot do an app purchase with it

If I read your post, you are already well on your way. . . . .

You have already taken the right steps 👍

Check again just to be sure:

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

1.

See -> Check your Apple ID balance

Check your Apple ID balance – Apple Support (UK)

(2)

Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

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

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

Apple often requires you to have a credit card on file even though you have an account balance. When you make a purchase, normally, the account balance will be used before the credit card is charged.