Unity doesn’t have an natural scrolling option on 18.04. One can enable that with libinput
To enable Natural Scrolling in
libinput addOption "NaturalScrolling" "true"
to /usr/share/X11/xorg.conf.d/40-libinput.conf.
It should be added before EndSection to the touchpad section.
Unity desktop GUI app can’t use libinput options.
You can switch to synaptics by
sudo apt remove xserver-xorg-input-libinput
sudo apt install xserver-xorg-input-all