Hello. I bought a key for the game Mount & Blade: With Fire & Sword, I enter the activation code, writes Code not found. What can be done ?

What do you mean you "bought a key"?

GOG don't sell keys.

Get a refund from whatever key reseller you used. Humble Bundle is pretty much the only legitimate source where you can buy GOG keys, not counting Amazon (sometimes offered with a Prime subscription), and retail boxed editions found in stores.

Congratulations, you got grifted.

I used to always buy keys at other sites and there were no problems. Why is technical support ignoring my ticket ?

They're not, they are just out of manpower.
Do not write a new ticket, but write a follow up to the existing one. That sometimes speeds things up.

But if you bought an invalid key from a third party, there's not much they can do really. Then you need to contact the store where you bought it from, it's then who are responsible for giving you working keys.

Apart from a handfull of games on Humble, there are no official gOg key sellers. This means that the keys that are sold are not legitimalty got in the first place, or are part of scams. If those key are discovered, they are revoked and therefore no longer works. the only thing you can do is go to the site where you bought the key from and ask for a refund.

You need to be more careful. Key reseller sites aren't reliable, and whilst you may come across keys people have legitimately bought on sale for trading, you are swimming in the same waters as scammers and thieves who purchase stuff with stolen credit cards or resell used codes.

GOG keys are available only from GOG (bought as gifts) or from Humble Bundle (a very limited selection of titles). That's it. Everywhere else are private key resellers and aren't supported nor guaranteed.

So if you buy a code elsewhere and it's useless, or you get as far as redeeming it and your game is revoked later that's not GOG's fault. Stop buying from key resellers and you shouldn't have a problem here.

EDIT: That being said there's a very rare occasion a GOG gift key purchased here has a hiccup with redeeming. Only then is it really GOG's support you have to contact.

LOL as in Biden Grifters Inc, godfathered by the now senile "The Big Guy", aka Liar-in-Chief, who was always dumb and stubborn as a mule. His old man peddled used cars for 20 years before devising the scheme of peddling Joe Jr's political influence for the whole clan to grift off of.

Fanatical and Groupees have also had some GOG keys.
Whether there will be any in the future is a different question.

Also, Prime Gaming gives GOG keys quite regularly these days.

"There were no problems". Until, one day, there are. Third-party sites are a gamble. A gamble with very good chances for you, but still not 100%. If the key doesn't work, you'll need to contact the site that you bought the key from, because GOG won't help you with that.

Publishers/Developers and of course GOG themselves can issue keys, which is how Amazon gets their GOG-Keys for the Prime Gaming offers. Other legitimate stores like Gamesplanet also source their keys from Publishers/Developers ( https://us.gamesplanet.com/support ), so you can get GOG keys that are legit even outside of GOG. Though there is not much point in that outside of site specific sales as the prices by nature of the source will be virtually the same as on GOG.

This is of course different from grey market reseller sites. Pretty much as soon as the platform is not your direct business partner but a mediator akin to eBay or Amazon Marketplace, you should stay away. Unfortunately many resellers bury their mediator role deep inside their FAQs or TOCs to appear legitimate where they are not.

Of course, I missed discussing those. I hadn't considered Gamesplanet. What I did indeed mean is "keys from stores" (the way the OP describes obtaining their key) which is why Prime Gaming didn't get included since it's issued via subscription not purchase.

Haven't seen this for a while, last i remember there was discussions on G2A i believe and this very problem, and the grey key sites... Yes get a refund. If they won't offer it, a chargeback on credit card may work, or paypal you can request it, include proof of the product failure(s) (though Paypal may not cover it, as they also have Ebay and refuse digital goods on Ebay).

How can this be? Well long story short, stolen credit cards (or other payment methods) and thieves buy a bunch of games gift cards and other things as 'gifts', then sell them on sites (a form of money laundering). When the credit cards are canceled and purchases reversed the keys suddenly are invalidated, and games you may have had redeemed may suddenly be yanked. The result? You lose; The devs lose; And the legit sites lose.

So what do you do? Trust keys from either official key sellers (Gog games from GoG, steam games from Steam, Humble Bundle, etc), from friends you trust, where the codes are not going to have a chance of being potentially stolen or purchased via stolen assets.

There are devs and companies that say they would rather you download and not pay for it rather than the charges and bad press associated with all of this.

But that should be the whole thing in a nutshell.