Keys can only be used once. If your key has already been used, support will not be able to free up the key or move it to another account.
There may be a chance that you have more than one account, so please be sure to check any other Ubisoft accounts you may have.
Please contact the retailer you bought your key from if it is already in use.
If you need help checking which account your key is registered to, please get in touch, including your key in the ticket.