iTunes gift card not valid how do I get my gift card to work ?

I’ve been to the retailer where I brought it from and they say it’s not their problem and that it is Apple’s problem.

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

👉Card is not valid

If you see a message that says that your card is not valid, here's what to do:

1. Make sure that your card is not an Apple Store Gift Card. Apple Store Gift Cards are gray, white, silver, or gold. You can’t redeem Apple Store Gift Cards in the App Store or iTunes Store. Find out what to do if you have an Apple Store Gift Card or don't know what type of gift card you have.
2. Try entering your redemption code manually. When you redeem a gift card, tap or click "You can also enter your code manually." Make sure that you enter the redemption code that begins with "X", not a different number on the card.
3. Make sure that you enter the correct characters. Some letters and numbers look alike, such as:

• B and 8
• D and O
• E and 3
• G and 6
• O and Q
• O and 0
• S and 5
• U and V
• Z and 2

👉 Card has already been redeemed

If you see a message that says that your card has already been redeemed, you might have already redeemed it. Sign out of the App Store or iTunes Store on all of your devices, then sign back in. This refreshes your Apple Account balance. Learn how to check your Apple Account balance.

If you need more help, contact Apple Support.

👉 Card is not activated

If you see a message that says that your card has not been properly activated, it might not have been activated by the retailer who sold the card. The best option is to go back to the retailer and try to resolve it with them.

If you need more help, contact Apple Support.

And read on the article.

Se also -> About Gift Card Scams

About Gift Card Scams - Official Apple Support