Troubleshooting Tip: Unable to pick channel 120 on wireless 5ghz in Canada Canada, Mexico, Australia, Argentina and Christmas Island

The reason why these channels are not selectable is they are reserved for Weather radar.

Weather radar is used to tell what the weather system will be, here is the link for more info on Weather Radar:

What is weather radar? The ultimate guide

Here is how to select the channels on the GUI:

WiFi Controller -> FortiAP Profiles -> Select one scroll to radio 2 5ghz select Set Channels.

The CLI config for selecting channels:

config wireless-controller wtp-profile
    edit "fortiapfun"

        config radio-2
            Alouette-kvm96 (radio-2) # set channel
*wireless_channel <36,40,44,48,52*,56*,60*,64*,100*,104*,108*,112*,116*,132*,136*,140*,144*,149,153,157,161,165>  
        end
    next
end          

It is not selectable on the CLI.