This is not an officially supported way to remove or disable the FortiClient software but it has worked for me. Also please be very careful deleting system files as any incorrect deletions could cause more problems on the machine.
Go to /Library/LaunchAgents and remove/delete any .plist files that have the name FortiClient in them. Do the same in the /Library/LaunchDaemons directory.
Reboot the Mac and you should be able to run the uninstaller for FortiClient to fully remove the remaining components and attempt a reinstallation.
If you want a more official way of troubleshooting this and fixing it I would suggest speaking to TAC.