Hmm/ Okay Selmf, I will try to sort it out. I'll keep you posted - Jason

SOLVED! There's currently a bug using Gdebi in Ubuntu 18.04 (March 2018) so I researched the solution:

In Terminal, type:
sudo dpkg -i (then drag and drop the .deb file directly in the Terminal window, so it creates a direct link to the .deb file) then sudo apt-get update - voila! solved! I installed the libunarr dependency, then the dev.deb, then the installer for 9.0.0