Is the copy of the game you bought a second-hand copy? Some retailers sadly pass off second hand games as new software - normally this isn't a problem, but for MMOs it means the player is stuck in a no-win situation and end up with a useless piece of software. If it is, it's likely that version was already used to make an account, hence it's registration code has already been registered with SE. Unfortunately there's not much that can be done in such a situation, other than tracking down a brand-new (unopened) version.

There was also an issue at launch where some registration codes weren't working, and I think SE asked those affected to contact them directly to sort out the issue, but I can't remember the details sadly.