Karabiner-Elements 10.22 on MacOS Yosemite. If you can't seem to assign a key to FN

Hi! If you’re using MacOS Yosemite and want to turn up/down the volume using just one hand (while mixing in a Cubase version suited for this OS), you can use a software called Karabiner-Elements.

The thing is—and I didn’t know about this 'till now—you have to take an additional step towards this goal (compared to other newer Karabiner-Elements versions). After you have mapped your other key (say Option_R), you need to scroll down to the following item and tick the last option:
Change F1…F19… > Change F1…F19 Key > Fn+F1…F12 to Functional Keys…
If your FN key is nonfunctional, you can tick everything from this section.