Hello everyone!

I want to activate a game on GOG and when I do it, I have a request error and I cannot activate the game. Does anyone know how to fix it?

Hello,

You mean you can't redeem a code? Open a support ticket.

Unless you bought the key from a third-party greymarket, in which case get a refund and buy it legitimately instead.

1) Which game?
2) Where did you purchase said game (key)?

That's most probably where your problem lies.
Key-resellers are notorious for illegally obtained keys (purchased via stolen CC).

Try to get your money back from the seller.
Buy your GOG games from GOG itself in the future.