After upgrading from Ubuntu 17.10 to 18.04 I am not able to login to the desktop environment. This screen shows after restarting:
Your OS is not properly installed. Try the following:
exec sudo -i dpkg --configure -a apt-get -f install apt-get update apt-get dist-upgrade apt-get install --reinstall ubuntu-desktop apt-get autoremove apt-get clean
If it doesn't work, reinstall Ubuntu. Follow this to reinstall the OS without loosing data. The post is about Ubuntu 16.04 LTS but it'll also work for Ubuntu 18.04 LTS.
In my case
sudo apt install lightdm
solved the problem.