Open a Terminal Check by below command 

     xrandr -q | grep " connected" 

    Check by the above command if the output looks like the below then go to step 2 directly otherwise follow the steps -1 and step 2

    You can check brightness with the below command 

    xrandr --output HDMI-1 --brightness 0.5


    eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 309mm x 174mm

    HDMI-1 connected 1920x1080+1920+0 (normal left inverted right x axis y axis) 517mm x 323mm


    Step 1

    Enter this command sudo gedit /etc/gdm3/custom.conf

    • Find the line #WaylandEnable=false in the configuration file
    • Remove the # from the above line – it should now read WaylandEnable=false.
    • Save the file
    • Reboot your system

    After the reboot, your Ubuntu 21.04 will reboot with Xorg. The Xrandr command and brightness-controller app will now work as it used to be on earlier Ubuntu versions.

    Step 2

    sudo add-apt-repository ppa:apandada1/brightness-controller

    sudo apt-get update

    sudo apt-get install brightness-controller