Unable to establish the VPN connection. The VPN server may be unreachable. (-8)

Verify the machine trying to connect is resolving any DNS you're using for the gate. Also ensure the machine itself has the ability to connect to your gate with DNS or IP using the correctly configured port setup on the gate (aka has internet). It's also worth restarting the machine as the vanilla FortiClient is oddly very unstable and a reboot has fixed these messages over the years for whatever reason.