Re: Forticlient VPN fails to install
This is might be something related to the leftover files from the previous client, I am assuming you can't update 6.0 to 6.4 directly since it was an enhached version.
If I were you, I will consider uninstalling the 6.0 first then have a fresh installation of 6.4.
Consider using Revo uninstaller to uninstall and delete all the leftover files then install the new version after rebooting the PC.
Also, you might have it failed at the first time, you have to go to control panel,remove add/remove programs , select the client then right click and choose repair.