Can you please try the following:
sudo nano /etc/default/grub
If the above produces an empty file instead of opening the grub file, try:
sudo nano /etc/default/grub.ucf-dist
Change GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" to instead say, GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
Press ctrl+x to exit, then the y key to say Yes to Save, then the enter key to save as current configuration. The terminal will revert to normal.
Next, update grub:
sudo update-grub
Reboot and test.
If you have a Lenovo Thinkpad, use this, instead:
GRUB_CMDLINE_LINUX_DEFAULT="acpi_osi=linux thinkpad-acpi.brightness_enable=1"
If none of the above work, you might try another brightness controller than the default one:
sudo add-apt-repository ppa:apandada1/brightness-controller
sudo apt update && sudo apt install brightness-controller