You do not need to create a new Ubisoft Account. If you are receiving this error, chances are you already have more than one Ubisoft Account. You can simply log in to your other Ubisoft Account(s) to find which one has the game registered.
Games purchased through Steam will appear in the Games tab in the Uplay PC application. If you are trying to play a game originally purchased through Steam, you can either launch the game through the Steam application or through Uplay.
1. Start by launching the Uplay PC client (Start -> Programs -> Ubisoft -> Uplay)
2. Click the Account Avatar at top right then select Log out from the list.
3. Login with a different Ubisoft Account.
If you need assistance determining which Ubisoft Account your game key is registered to, you will want to create a support ticket.
Note: Please be sure to include your game's CD Key / Activation Code when submitting a ticket.