A better solution would be to install the 'lubuntu-core' package which is a meta-package and try the release upgrade again. From Mainlog.txt we can see:
2018-05-04 01:57:33,036 DEBUG none of the '['ubuntu-desktop', 'kubuntu-desktop', 'xubuntu-core', 'xubuntu-desktop', 'ubuntustudio-desktop', 'mythbuntu-desktop', 'kubuntu-netbook', 'lubuntu-core', 'lubuntu-desktop', 'ubuntu-gnome-desktop', 'ubuntu-mate-core', 'ubuntu-mate-cloudtop', 'ubuntu-mate-desktop', 'ubuntukylin-desktop']' meta-pkgs installed 2018-05-04 01:57:33,037 DEBUG guessing 'lubuntu-core' as missing meta-pkg 2018-05-04 01:57:33,085 DEBUG marked_install: 'lubuntu-core' -> 'True' -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1768949 Title: cannot upgrade To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1768949/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs