First of all, where did you buy this "redeem code"?

Where did you acquire the code? Aside from a couple of games on Humble, GOG is the only place to legitimately buy GOG games. If you bought the code at g2a or some such place, it is possible the game was acquired with a stolen credit card and the code has since been cancelled.

If you did buy it from a 3rd party, you should contact them for a refund and buy the game directly from GOG.

Ye I bought it from G2A, I tried to refound it buy they said that I should talk to GOG about the problem, but since they havent answeared I looked for help here.

EDIT: Nvm, see above

I hope your problem resolves just fine and you get your game but just don't buy on g2a man, anything goes there, it's not an official reseller.

"some error" is caused when you try to redeem a code that has already been refunded to original account.
May be you got a code that was brought by fraud from G2A reseller and GOG already refunded money to original card owner.

My advice is stay away from G2A.
They mostly sell stolen codes.
If you want to play games without worry then buy games directly from GOG during sale.