Comment 2 for bug 1450044

Indeed the log says

апр. 29 15:48:07 username-pc systemd-backlight[442]: Failed to get backlight or LED device 'backlight:acpi_video0': No such file or directory

Can you please give me the output of

    grep -r . /sys/class/backlight/*

Are you using the free driver or the AMD proprietary one? On several boards the backlight is directly hardwired to the brightness buttons and they can't be controlled through software. If you press the brightness keys, do you get a notification bubble for that which shows the current value?