I just took a look at your profile. It's because you already own the game, it's just some "Upgrade". Here's a tutorial to activate the key.

https://support.505games.com/support/solutions/articles/150000146873

But are you sure, you want the game, since you already have it and didn't play? ;)
Just let me know, if the tutorial works or if you want me to do a reroll. Thank you!