I was in the same situation, after looking at the Wolves of Steel "modpack".
You can only activate on 1 UPlay account, so you would need to get the info from the existing one.
Contact Uplay Support by making a new ticket which contains your CD key and a screengrab of the error (cd key in use).
Info Here:
https://support.ubi.com/en-gb/FAQ.as...0000000eTCxCAM