Unable to adjust display brightness

Hi,
I recently installed Zorin OS 16 but have been unable to adjust the display brightness. I can see and interact with the brightness slider, but it does not adjust the brightness of the screen. Can I get some help with this?
Thanks!

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

haven't tried myself the solution proposed by Aravisian but do you know if this would work with an external monitor? Just curious.
Thanks

I had this same issue. Installed another brightness app which did the trick

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It should work the same for all monitors.

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