There seems to be a issue in adjusting the screen brightness in ubuntu bionic beaver version 18.04. The shortcut keys do not work for me and I always use the terminal to adjust the brightness. Please send a software update to fix the issue.
My screen brightness works properly in 18.04, so this does not seem a universal problem.
Please file a proper bug report, in the correct venue, and with enough information about your system to localize the issue.