the error says it on screen: Cipher mismatch. The FortiGate and the Browser do not speak the same security protocol and fail to negotiate a TLS/SSL tunnel.
Either your browser or FortiGate is ancient or your FortiGate is set up to not use strong-crypto or only allow older TLS versions.
On CLI you can search for it:
show | grep crypto -f
show | grep TLS -f
show | grep SSL -f
Respective options might have to be changed. A good idea to do so. Above error message only appears when one of the clients isn't speaking up-to-par encryption standards for some reason.
