can not activate the game

So I have bought the game for £5 from steam, and I can not activate it, when I run the game it gives me my serial number, and when I copy it, and paste itminto the serial number box, it says that its invalid, but at the top of the window it says GC2 Twilight of The Arnor, and this happens every time even when I have restated my computer re installed the game few times, it still gives me this message, any fixes?