Are you sure you are in the right account? The code links to the account it's first installed with, so you can't re-register.
Give these a try:
The game no longer shows up in Origin
Try the following things:
- Restart Origin.
- Log out and back in and use the email address instead of the username. Make sure to use the one you used to register the game in the first place.
- Reinstall Origin.
- Restart your router/modem.
- Make sure the game isn't hidden by accident. To check, put the filter on Hidden Games. If it's there, right-click on the game to unhide ((click on the spoiler below to view an exemplary screenshot))
- Get in touch with an EA Game Advisor.
Source: http://answers.ea.com/t5/The-Sims-4/FAQ-Serial-Codes-Game-Activation-and-Download-Issues/td-p/4228232
