I recently ran into an issue where my FortiClient app on macOS no longer worked and I had to reinstall it. Unfortunately, if the installer detects it is already installed, it refuses to install and instead, simply attempts to update it, concludes there's no new version, and ends with the message "No updates found". The official solution is to use the FortiClient Uninstaller app, which can also be found in the /Applications, but if that's no option, you're seemingly stuck.
Fortunately, there's an easy way out by manually removing the app files, then rerunning the installer.
First, using Finder, completely remove the following apps (remove and remove from Bin):
Start Terminal and remove these folders:
sudo rm -rf /Library/Application Support/Fortinet/ rm -rf Library/Application Support/Fortinet/ rm -rf Library/Application Support/FortiClient/
You can now rerun the installer. As it no longer detects the app and support files, it will now continue with the download and installation.