I have an iTunes card and it won’t work

Check 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

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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

If you have a valid payment method on file, instead of buying or order an App Store & iTunes gift card, you can Add money to your Apple Account balance anytime, no gift card needed, see -> Apple Gift Card - Apple (UK)