Display brightness intensity level is adjustable but has no effect on screen. How reproducible – Steps to Reproduce: 1. From desktop top panel select Applications → Settings → Power-manager; in Display tab, turn on feature Display power management. 2. From desktop panel, adjust brightness.
This may mean your hardware is not supported by the Linux kernel. You can try to enable / disable the handling of brightness keys by xfpm but in the end if your hardware is not supported by the drivers there is nothing we can do about it. You can also try to switch to a different display driver (e.g. nouveau instead of nvidia).