When you search for "forticlient 5.6 download offline installer" , you are specifically avoiding the online web installer (a small .exe file that downloads components during setup). Here is why the offline version matters:
msiexec /i "FortiClient_5.6.10_x64.msi" /quiet ADDLOCAL=ALL DISABLEAUTOUPDATE=1 forticlient 5.6 download offline installer
Ensure that no other antivirus or VPN software is running. Conflicts between VPN clients are common and can cause the installation to fail. When you search for "forticlient 5