On an Ubuntu 18.10 system I installed all available updates with update-manager and then ran 'do-release-upgrade -d' and was told I needed to install even more updates (gnome-software) before I could upgrade. I installed ubuntu-release-upgrader-gtk from -proposed and tried the process again w/o -proposed enabled. This time I was able to upgrade to the Disco Dingo!
On an Ubuntu 18.10 system I installed all available updates with update-manager and then ran 'do-release-upgrade -d' and was told I needed to install even more updates (gnome-software) before I could upgrade. I installed ubuntu- release- upgrader- gtk from -proposed and tried the process again w/o -proposed enabled. This time I was able to upgrade to the Disco Dingo!