Screen Brightness not Changing on Dell XPS 7590

I recommend using the ICC Brightness tool, which worked well for my Dell 7590 with OLED screen. It requires compiling and installing a small utility, but it works well.

Full instructions can be found at https://github.com/udifuchs/icc-brightness but here is a summary.

$ sudo apt install git liblcms2-dev
$ mkdir ~/git
$ cd ~/git
$ git clone https://github.com/udifuchs/icc-brightness
$ cd icc-brightness
$ make
$ sudo make install

After a reboot your brightness keys should now work. Be careful not to use this in conjuction with the xrandr solutions as they don't play nicely together.

Enter xrandr and check the output. In my case it was eDP-1. To test if it is the right one, enter xrandr --output eDP-1 --brightness 0.6 (intel driver). If yes, the brightness should have changed.

I found it at How to install Ubuntu on a Dell XPS 15 2019.