Sims 4 Get To Work - Brand new: Says that the code has already been used!! Help

Contact EA:
http://help.ea.com/en/contact-us/new/

You can only use the code for a game once to link it to an Origin account. When you have the base game on one and the expansion on the other it won't work. 
So you'll need to ask EA to move the game, but you´ll need to prove you are the owner of both accounts and games.