Online Gift Card for wrong App Store

Sorry, but I'm afraid, the official Apple Support can not help in this case:

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If you see a message that says the code must be redeemed in another storefront, the card was probably purchased in another country or region. You can't redeem Apple Gift Cards, App Store Cards, or App Store & iTunes Gift Cards outside of the country or region of purchase. For example, if you bought an Apple Gift Card in France, you can't redeem it in the App Store in the United States. If you have a physical gift card, you can return to the retailer where it was purchased.

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copied from: If you can't redeem your iTunes Gift Card or code - Apple Support

Did you contact the person or shop who sold you the Apple Gift card?

If you have a physical gift card, you can return to the retailer where it was purchased.

I am truly surprised by that statement. Unless the retailer has some way of scanning the code on the card to see if it has not already been redeemed, I don't know how the retailer could tell that the card wasn't redeemed already and somebody is trying to commit fraud in asking for a refund. Past advice on this forum has been if you receive a card that you cannot redeem in your country you should find somebody else who can use it.

Incidentally, this Apple web page says you cannot return gift cards:

https://www.apple.com/shop/help/returns_refund

There are some items, however, that are ineligible for return, including:

• Apple Gift Cards (includes previous generation Apple Store Gift Cards and App Store & iTunes Gift Cards)