Enable Fn-Brightness Buttons Ubuntu

Solution 1:

This is not how Fn keys work. If you press Fn+5, it is not going to send any special signal to the system. It is only going to send 5 as the keystroke.

The thing to understand here is that Fn keys are hardcoded in your laptop keyboard. So, if you press Fn key with, let's say F5 key, and if F5 key has a small icon below it marked as mute, then it is going to send a special signal (which will mute your speakers, if everything is properly set).

If you combine your Fn key with any other key on the keyboard which have no special markings for Fn key, it is not going to send any special signal.

Solution 2:

Can you set your brightness without using keyboard shortcuts? If yes it's not a grub or acpi issue.

You actually need to set keybindings to brightness functions, common media keybindings are included by default, but in your case your brightness keybindings are not. Configure them by using Dconf Editor.

https://wiki.ubuntu.com/Keybindings