Find out which launcher owns the key first
A key is issued for one platform and is inert everywhere else. Before redeeming anything, confirm from the order confirmation — not from the game — which launcher the code belongs to. A game sold on Steam, Epic and the EA App has three separate keys, and the code you were sent is only one of them.
Codes are single-use and most launchers count failed attempts. If a code is rejected, stop and read the message rather than retrying it on another platform; repeated failures can trigger a temporary block on redeeming any code on that account, which turns a small problem into a wait.
PC launchers
Steam: in the desktop client, Games → Activate a Product on Steam. Steam keys can carry region restrictions, and the client says so explicitly when one applies.
Epic Games Store: your account's Redeem Code page, reachable from the account menu in the launcher or on the web.
EA App: the account menu's redeem option. Older Origin codes generally still redeem here, since the account is the same.
Ubisoft Connect: the account menu carries an activation entry for keys.
Battle.net: the account settings redeem page. Blizzard codes are usually region-bound by account.
GOG: the account settings redeem page. GOG games are DRM-free once installed, though redeeming the code still binds the licence to an account.
Microsoft Store PC titles: either the Store app's Redeem code option or Microsoft's redeem page on the web — both land on the same Microsoft account.
Consoles
Xbox: on the console, Store → Redeem code, or on the web through the Microsoft account redeem page. The code redeems against the Microsoft account, so the console you use does not matter — the account signed into it does.
PlayStation: on the console, PlayStation Store → Redeem Codes, or through the web store while signed in. PSN codes are tied to the store region of the account.
Nintendo: in the eShop on the console, or through the Nintendo account website. Nintendo codes are strictly region-locked to the account's country and there is no workaround worth attempting.
On all three, the account region is what the code is checked against. A code bought for one country's store will not redeem on an account registered elsewhere, and console account regions are harder to change than PC ones.
Games that need a second account
Some publishers require their own account on top of the launcher you bought through, and the key redeems into one while the game asks you to sign into the other. Ubisoft and EA titles bought on Steam are the usual examples: the key activates on Steam, but the game will not start until a Ubisoft Connect or EA account exists and is linked.
This is not a fault and not a sign of a bad key, but it does mean an activation that "worked" can still leave you unable to play for another ten minutes. It also means the account holding the entitlement may not be the one you expected, which matters if you ever intend to move machines or hand a family PC on.
If a game insists on an account you do not have, create it before assuming the key failed. The error at that stage is about the linked account, not the code you redeemed.
Windows and Office
Windows: Settings → System → Activation, then change or enter a product key. A licence that activates is not automatically a licence that transfers — OEM licences are tied to the machine they first activate on, retail licences are not, and listings frequently do not distinguish. If you expect to move the licence to a future PC, buy retail and keep the proof of purchase.
The failure worth knowing about on very cheap Windows listings is the volume licence. Keys intended for organisations can activate on a home machine and then stop being valid later, because the licence was never issued for that use and the activation can be withdrawn centrally. An activated Windows is not proof of a licence you are entitled to keep, which is the whole difficulty with the cheapest end of this market.
Office: perpetual versions redeem through Microsoft's setup page against a Microsoft account, which then owns the licence. Microsoft 365 codes extend an existing subscription of the same plan rather than stacking a second one alongside it, and there is a ceiling on how far ahead a subscription can be extended — so check the remaining term and the plan before buying another year.
For both, the account you redeem against matters more than the machine you install on. Redeem to an account you control and will keep.
Gift cards and wallet codes
Wallet codes add funds rather than granting a product, and they redeem in the same place as game codes on each platform — Steam's client, the PlayStation or Xbox store, the Nintendo eShop.
The constraint is currency and account region, not location. A code must match the currency of the wallet it is going into, and it must match the store region of the account. Neither can be worked around after purchase, and a redeemed code cannot be reversed, which makes this the one category where checking twice before redeeming genuinely matters.
When activation fails: reading the error
"This code is not valid in your region" or similar: the key is region-locked and your account is outside it. The key is fine; it is not fine for you. This is a refund conversation with the store, and a quick one if you have the error text.
"This code has already been used": someone redeemed it before you. Open a dispute immediately — this is the easiest failure to evidence and usually the fastest to resolve.
"This code is not valid" with no further detail: most often the wrong launcher, or a code typed with a character transposed. Copy and paste rather than typing, and confirm the platform against the order before escalating.
Whatever the message, screenshot it. Every store's dispute process moves faster with the exact error than with a description of it, and the refund window is usually short enough that a day spent rephrasing the problem is a day you cannot spare.