keyboard shortcuts is missing volume options

I'm running high sierra on an older iMac and I've just started using a magic keyboard but for some reason I don't have a "function keys" section in keyboard shortcuts. Also, there is NOT a pre-existing option for me to assign a shortcut for volume buttons and they're not working on their own. I've attached some photos to show what my system settings boxes look like & any help would be greatly appreciated!

Thank you,

Bird