Hi, i bought The Witcher 1 Enhanced edition in box and there is cd-key which i can redeem on my gog account, but when i wrote this key to reedem window it says me "Oops, something went wrong. Please this page and try again. If you're here about code MMLXXVII, please be aware that it is invalid and not part of an official promotion." so i dont know what to do, yes i can play this game without this but when i have the key i want have this game on my gog account, whats the problem?

Contact support.

Did you buy it second hand? If so, someone may have already redeemed the key. Like Hickory suggested the best thing to do would be to contact support, they'll be able to tell you for sure if the game has already been claimed.