I added my gift card into my account and it won’t let me use it to purchase anything

Do you also have still a payment method on your account? Apple says: "Some purchases might require that you have a credit card on file, even if you use your Apple ID balance to make the purchase." - from: What you can buy with your Apple ID balance --> https://support.apple.com/HT211187

For a variety of reasons Apple will almost always require a functional payment method such as a credit or debit card be present when downloading anything from the iTunes or App Stores that isn't totally free (so, for example, a "free trial" that turns into a paid subscription isn't ' free'), even if the item will be paid for with Apple Account balance. The only solution to satisfy this requirement is to add a valid payment method*.

This document explains how to 'Change, add, [review,] or remove Apple ID payment methods' on a Mac or Apple mobile devices; click here --> https://support.apple.com/HT201266

Change your Apple ID payment method if you don't have an Apple device --> https://support.apple.com/HT210755 - PC, MS Surface, Android, Chromebook, online at tv.apple.com

'If you can't remove your last payment method or use no payment method with your Apple ID' click here --> https://support.apple.com/HT203905

- When updating an old card and have ongoing subscriptions, or are the organizer of a Family Sharing group, you may need to first add the new card and make it the primary payment method before deleting the old card.

- A payment method is required if you're using your account with the App Store for the first time.

If you still have problems, click here --> https://support.apple.com/choose-country-region/media-services Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232

Additional resources:

*'Payment methods that you can use with your Apple ID' --> https://support.apple.com/HT202631

Also realize that you don't really have too much choice in how something is paid.

How App Store and iTunes Store purchases are billed. Click here --> https://support.apple.com/HT201359

"Which payment method is charged?

When you buy items or pay for subscriptions, Apple automatically attempts to charge your payment methods in this order:

1. Apple attempts to use your Apple ID balance to cover the full amount.

2. If you don't have Apple ID balance, you don't have enough to cover the full amount, or an item cannot be paid for with Apple ID balance, Apple attempts to charge your primary payment method (typically a credit or debit card, but varies according to country). The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list.

3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again.

and:

Some purchases aren’t charged to your Apple ID balance

- If you're in a Family Sharing group, purchases are charged to your personal Apple ID balance. If you don't have enough Apple ID balance to pay for the purchase, the remainder is charged to the family organizer's primary payment method.

- You can’t charge gifts to your Apple ID balance. If you send a gift or gift card, your primary payment method is charged.

- Some subscriptions might not be charged to your Apple ID balance.

Hello ~Take a look here:

What type of gift card do I have? - Apple Support

~Katana-San

My gift card started with y

and I can’t purchase it

I am playing a game and can not use my gift card account