cannot turn on keyboard backlight

To enable the backlight:

echo 2 | sudo tee /sys/class/leds/asus::kbd_backlight/brightness 

The 2 at echo 2 | can be changed to a value between 0 - 3, with 3 being the brightest.

To disable the backlight, enter:

echo 0 | sudo tee /sys/class/leds/asus::kbd_backlight/brightness

The path may vary depending on laptop model and your OS. For example Lenovo Thinkpad L390 running Manjaro has /sys/class/leds/tpacpi::kbd_backlight/brightness. You can use find to see the correct path:

find /sys/class/leds -name '*kbd_backlight'

Try

xset led on

or

xset led 3

This worked well with my CMSTORM (Cooler Master Storm Devastator) keyboard.

You can turn on backlight by this command

sudo tee /sys/class/leds/asus::kbd_backlight/brightness <<< 3

This will set it to maximum. The number at the end means brightness ( 0 - 3).

You can link this command to some hot key combination.

You can also read article regarding setting up Ambient Light Sensor.