I would recommend closing down Character Creator and iClone and restart your computer. Then try to install the Headshot Plug-in again.
However, if the issue is the same, please logout from the Hub and then log back in again making sure to use the correct account details that were used to purchase CC3 and the Headshot Plug-in. Hopefully this will resolve the issue, but if it doesn't, please contact Support using the link below. They will be able to advise you further.
https://www.reallusion.com/CustomerSupport/UserEx/QForm.html
Peter
Forum Administrator
www.reallusion.com