Thanks for the bug report.
There is no backlight control for desktops with external monitors.
Although in Xorg sessions you can use this to simulate it:
xrandr --output NAME --brightness VALUE
Also "the screen has pathches" sounds like bug 1867668. You need to
remove the old 'intel' driver and use the newer default 'modesetting'
driver instead.
** Package changed: xorg (Ubuntu) => xserver-xorg-video-intel (Ubuntu)
** Changed in: xserver-xorg-video-intel (Ubuntu)
Status: New => Invalid
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https://bugs.launchpad.net/bugs/1959179
Title:
Video driver not working properly
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