On Ubuntu 21.04 with a Dell Latitude 7550 I had problems with the function keys not being able to adjust the screen backlight levels. The laptop has a discrete NVIDIA GPU (along with the Intel on CPU GPU).
Edit the X11 NVIDIA configuration file: /usr/share/X11/xorg.conf.d/10-nvidia.conf
At the bottom of the file, add the following content:
Section "Device"
Identifier "Device0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
Option "RegistryDwords" "EnableBrightnessControl=1"
EndSection