I bet you have Steam Verison of the game...check to see if you have Steam Wallet in Mog Station account. If so your code from amazon will not work. I had the same issue. Trying to get refund now from SE store since i had to go purchase another copy on Steam. ) Also it should be 20 digit code. The 18 Digit code should of sent you to a website...that takes it and gives you back a 20 digit code.