You can also try to verify sysfs interfaces such as below example

$ ls /sys/class/backlight/
intel_backlight
$ cd /sys/class/backlight/intel_backlight
$ cat max_brightness 
120000
$echo 120000 | sudo tee brightness
$echo 60000 | sudo tee brightness

If this does not change brightness, something is wrong with VGA (or
BIOS) brightness interfaces.

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https://bugs.launchpad.net/bugs/1890055

Title:
  Brightness control does not work with 5.4.0-42.26

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