This would normally indicate that the key you have received has already been used by another account. Each CD key is unique and can only be used once. If your key has already been used, support is not able to free up the key or move it to another account.
There is always the possibility that you may have more than one Uplay account so please be sure to check any other Uplay accounts you may have. If you need help checking which account your key is registered to, please click HERE to submit a question to support. For security reasons, if you need help recovering your account you will be asked for your date of birth or other account details.
When submitting your ticket, please include the following:
Subject: Copy and Paste the following text '[Activation] Key Already in Use'
Question: Enter a description of the problem including when the game was purchased and where
Attach Documents to Question: Include a clear digital image of your key and your proof of purchase clearly showing the purchase date *
* When submitting a picture please ensure that the entire key is visible and not obscured. We also ask that it is a reasonable size, so no need to submit an image that is 1MB+ for example.