It's saying "mismatch" between your client device and the FGT. It's NOT saying the FGT doesn't support the encryption level your device is requiring. Go to CLI under "config system global" then "get | grep admin-https-ssl". You should get like below:
fg50e-utm (global) # get | grep admin-https-ssl
admin-https-ssl-versions: tlsv1-1 tlsv1-2 tlsv1-3
Then you need to make sure that the browser's supported TLS versions would overlap with them. Each browser would have different way to do the settings, which you need to figure out for your browser.
Many thanks for the prompt reply!
I have tried different browsers, they are support tls all versions.
A have tried enable all ssl options: