I am unable to use the gift card that I redeemed.

OK.

If you get a message -> Your purchase could not be completed, you need to Add a payment method to your Apple ID – Apple Support (UK)

• If you want to change your card, add first the new card, then remove the old one.
• or if you use your Apple Account balance, you need to set your Apple Account balance as your default payment.

See also -> 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, the amount will be credited to your (1) Apple Account balance and (2) 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 or App Store & iTunes gift card is automatically added to your Apple Account balance.

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.

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

if you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

Do you get any error message?

See this Apple article:  If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

check. ⤵️

Card is not valid

Card has already been redeemed

Card is not activated

Code is unreadable

Code must be redeemed in a different country or region

• You can't redeem Apple Gift Cards or App Store & iTunes Gift Cards outside the country or region of purchase. For example, if you bought an App Store & iTunes Gift Card in France, you can't redeem it in the App Store in the United States.

Contact Apple Support -> Contact Apple Support.

See also -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support

Try this link--->Check your gift card balance

"Check the balance of your App Store & iTunes gift card

After you redeem an App Store & iTunes gift card, learn how to check your Apple Account balance."

How to redeem your Apple Gift Card or App Store & iTunes gift card may provide additional information.

Can you help me when I try to reedem my credit card it just loads and never ends even with my good internet and without different I tried downloading the new update and nothing new happened j tried power of and on 7 times I tried doing it again 342 times

it just loads and never ends

Start here, check the network connection:

You need to have a good Service and/or Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

See also -> If you can't connect to the App Store, iTunes Store, or other Apple services

If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support

👉 If you tried all of the above steps and you still can’t connect, contact Apple Support.

👍

With pleasure to be able to help you.

Thank you for your feedback and you're welcome.