Can I use a US Nintendo eShop card on a European account?
No. A US Nintendo eShop card cannot be used on a European Nintendo Account. Nintendo eShop cards and download codes are region locked — they can only be redeemed on a Nintendo Account set to the same region as the card. A US card will only work on a Nintendo Account set to a country in the Americas, and a European card will only work on a Nintendo Account set to a European country. If you try to redeem a US card on a European account you will receive an error and the code will not work. To use a US eShop card you have two options:
Create a separate Nintendo Account set to a US region and redeem the card there — you can then use that account's balance to purchase content on that region's eShop
Contact the seller for a replacement card in the correct region for your account
Always make sure the eShop card or game code you purchase matches your Nintendo Account region before buying.