The most common redemption failure has nothing to do with the code itself.
Every platform account has a region, set when the account was created. Check it in account settings before you buy, not after.
Your wallet holds one currency. A euro card will not top up a pound wallet, even inside the same region.
Every Keeoto listing shows a region badge. Global codes work everywhere, EU codes work in EU accounts. Pick the badge that matches step one.
If a code is refused, the platform tells you why. Region and currency errors look different from already-used errors. Take a screenshot before you try again.
If the code did not match the listing badge, buyer protection covers it. If the badge was right and your account differs, the code can often be resold.