Because you are trying to add a new account code onto an account you already have. You bought something you shouldn't have. You need to use the upgrade option. A 1.0 player was already given the game for free, they told you not to buy the game again about a month ago. But a zillion people must have missed this too because you're not the only person to do it. Wait to see if SE changes their policy of not allowing you to use a new account code onto an already made account, or try to get a refund and use the upgrade option.