1. Have you installed the module and its dependencies as per the instructions
2. Did you restart MM after installing and changing the config.js file.

With the config above place at the end of modules section you should see the following lines in the log.

MMM-ModuleScheduler is removing all scheduled jobs
MMM-ModuleScheduler received CREATE_GLOBAL_SCHEDULE
MMM-ModuleScheduler is creating a global schedule for all modules using "0 6 * * *" and "0 22 * * *" with dim level 20
MMM-ModuleScheduler has created the global schedule for all modules
MMM-ModuleScheduler will next show all modules at Sun Jan 13 2019 06:00:00 GMT+0000 (UTC)
MMM-ModuleScheduler will next dim all modules at Sat Jan 12 2019 22:00:00 GMT+0000 (UTC)

Whilst testing, you could use the following config to toggle the brightness more frequently (i.e. every two minutes):

        {
            module: 'MMM-ModuleScheduler',
            config: {
                global_schedule: {from: '0-59/4 * * * *', to: '2-59/4 * * * *', dimLevel: '20' },
            }
        },

Let us know how you get on.

Once installed, adding something like this should to your config.js should work

    {
        module: 'MMM-ModuleScheduler',
        config: {
            // SHOW ALL MODULES AT FULL BRIGHTNESS AT 06:00 AND DIM THEM TO 20% AT 22:00
            global_schedule: {from: '0 6 * * *', to: '0 22 * * *', dimLevel: '20' },
        }
    },

pi@raspberrypi:~/MagicMirror/config $ xbacklight
RANDR Query Version returned error -1

no, doesn’t work in RPi3