This is a very big problem most people have with Ubuntu on their laptops. Me too I had this issue for a very long time until I found the solution 3 minutes ago
Actually, the issue I’m talking about is the fact of not being able to change the screen brightness (backlight) neither using Fn key nor using “Brightness and Lock” in system settings; and the solution is to change the brightness value in a specific file manually, this file is /sys/class/backlight/acpi_video0/brightness. To do that, you only need to run the following command, and the brightness will change instantly. Don’t forget to enter your root password whenever you’re asked to.
If you’ve got an Intel itegrated GPU :
echo 60 > /sys/class/backlight/intel_backlight/brightness
I had an ATI Radeon video card, and this used to work, but here, the highest value is 7 instead of 100:
echo 5 > /sys/class/backlight/acpi_video0/brightness