Gotcha. I tried both 16.04 and 18.04 when I wrote the guide. 16.04 wasn’t as complicated so I pushed ahead with that.
But later on I documented the extra steps here if you want to keep trying on that version. 18.04 uses more up-to-date repositories with extra layers of security. You’ll probably see why I opted to stick with 16.04 for the guides. I don’t think you’ll run into anything else, but I don’t run any of my servers on 18.04 so I wouldn’t really know.
Anyway, signing off for the day. Please, keep us posted with your progress over the week.