Keys for this game can only be activated one time. After the initial activation, the key can never be used again. So, this problem is only caused by one of two things:
1. If you previously activated the key and are being asked to enter it again because you are reinstalling the game, you are simply at the wrong login screen. Please follow the instructions in the following FAQ for assistance with getting to the correct login screen, where you can enter your Uplay account name and password:
- If you still receive this error message after entering your Uplay account name and password, most likely you are logging in with the wrong account. Unfortunately, we cannot give account information out over the web for security reasons, so you will need to contact us http://www.ubi.com/US/Support/InternationalSupport.aspx. One of our representatives will be able to verify the correct account information for you.
2. If you purchased the game used or from an unauthorized retailer like eBay, the key most likely has already been used by another person. If this is the case, you will need to return the game to the retailer and exchange it for a new copy or ask for a refund.