I recently bought the Game and when I try to create my Eugen account it says "Key Invalid" I know how to get the serial key for the game you have to go to the Browser and go to Games, Click on "Steel Division 2", Press "More" and then click on "Serial Keys" There I see the serial key for the game but when I use the Serial Key to create my account it says my key is "Invalid" despite me having legitimately bought the game. I have informed support but of course they have not responded for weeks now. Is there any way I can fix this?

Yeah, I have the exact same problem and was trying to find a solution for weeks now but nothing works.
I have however found this on their website, here is a link for you if you want to check it out https: //steeldivision2.com/buy/

It might have to do something with the fact that GOG is not listed as an "authorized store", but if that is the case this is bs and I want my money back, since I payed for a game that I cannot fully play.

Same here, invalid CD Key. Nothing i can do to fix that right now

didn't work for me at the beginning (first 12 hours after purchase). but after 5 attempt of creating account it accepted the key

Same problem here.

Then I read somewhere that you shouldn't copy/paste the key.
So I tried to type each character one by one, and it worked.

Strange? Yes.