Greetings to the entire community from Argentina...
I tell you that I am controlling the screen brightness on the Budgie desktop with commands from the terminal.
I have installed the brightness controller applet and nothing happens. No, if the same thing happens to you, I would like to find a solution. Thanks for your attention.

Just tried and working fine for me. Make sure you have a fully up to date system. You can do it from Software Center of from command line sudo eopkg up -y. Then you can do system check for broken packages:
sudo eopkg check | grep Broken | awk '{print $4}' | xargs sudo eopkg it --reinstall