If a gift card does not work, this is why, about nine times in ten. It takes thirty seconds to avoid and cannot be fixed afterwards, so it is worth those thirty seconds.
What is actually happening
A gift card is issued by a brand's operation in one specific market and is a claim against that entity. Amazon's US business and Amazon's German business are different companies, settling in different currencies, with different catalogues and prices. A code issued by one is not something the other can honour — not as a policy choice, but because there is no mechanism for it.
So when amazon.de says your code is invalid, it is not broken. It is a valid claim against a different company.
Check the account, not yourself
This is the part people get wrong. The lock follows the account redeeming the code, not your physical location.
A German who moved to Spain, still using a German Amazon account, needs the German code. Someone travelling in Thailand with a US Steam account needs the US code. Where you are sitting is irrelevant.
So: open the brand's site, find the account's country or store-region setting, and buy for that.
Brand by brand
Amazon. Every national store is separate. We list US and DE; ask if you need another and we will usually source it.
Steam. The subtle one. A wallet code is tied to the store region of the account, and Steam will also refuse a balance in a currency that does not match. If your account's region is stale because you moved, buy for the stale region — or change the account region first, which Steam permits roughly once a quarter and only from the country you are actually in.
PlayStation, Xbox, Nintendo. All strictly locked to the account's country, and all three make changing a console account's country hard once a balance exists. Get this right the first time.
Apple. Locked to the App Store country of the Apple Account. Changing that country requires spending the existing balance to exactly zero first, which is a real obstacle rather than a formality.
Google Play. Locked to the Play country, which is set by the payment method on file and can usually only be changed once a year.
Netflix, Spotify, Disney+, HBO Max. Locked to the billing country of the subscription.
Genuinely global: Roblox, Discord Nitro and Booking.com. We mark these "Global" rather than with a country code.
Buying for someone else
Ask them what region their account is. Do not infer it from where they live — that is precisely the assumption that fails. A code is a bearer instrument, so once you have the right region you can simply send them the code.
If you bought the wrong one
Tell us immediately and do not attempt to redeem it. An unredeemed code can sometimes be exchanged, depending on the brand and how recently it was issued. A redeemed code is spent, and no amount of goodwill on our side recovers it.
We will tell you which of those two situations you are in straight away rather than leaving you hoping.