Quick answer: A US eShop card only redeems on a Nintendo Account whose country is set to the United States. The fix is to redeem the code on a US account — the Switch console itself isn’t region-locked, only the account is.
This message means the card's region doesn't match the country set on the account you're trying to redeem it on. It's one of the most common eShop card hiccups, and it's easy to fix.
Each region's eShop has its own currency and storefront, so codes are tied to the region they were issued for. A US eShop card adds US dollars to a US account and buys from the US eShop. If your account's country is set to somewhere else, the US code is rejected.
The Nintendo Switch console is region-free, so the only thing that has to match is the account. Lots of buyers outside the US keep a dedicated US account just for US eShop cards.
eShop funds don't transfer between regions, so a US code can't be added to a non-US balance. Redeem it on a US account instead. For the full picture, see our region & compatibility guide.