Ubuntu 18.04 Server does not come with a desktop by default, but any of the normally available desktops can be installed with the tasksel util.
sudo apt-get install tasksel
List available desktop installations (and other installs/collection of packages):
tasksel --list-tasks
Select required desktop from displayed list and install, e.g.:
sudo tasksel install lubuntu-desktop
Wait for install to complete, reboot, done.