when i start pc give mi this error
Please have a look at:
https://wiki.archlinux.org/index.php/Backlight#Kernel_command-line_options
and for how to add kernel boot parameters:
For a similar issue, see:
Hope this helps!
I don’t know what your hardware is but on mine i just added acpi_backlight=video as @pebcak has shown with the info above.