I purchased Antivirus Plus 2017. In Central my subscription is active, but when I try to add my computer, I get the message "The key was already used to generate a subscription." I have logged out and reinstalled, but am having the same issue.
You can use the Bitdefender uninstall tool to get rid to some unwanted leftover on the machine.
Reinstall this (go to Central) to refresh everything.
If issue still persists, please contact support: https://www.bitdefender.com/support/contact-us.html