Server and minimal installs are normally headless and have no graphical interface.
If needed you can add a GUI manually. The process is slightly different depending on the distro.
RedHat / CentOS 7.x
# yum update
# yum groupinstall "Server with GUI"
RedHat / CentOS 8.x
# dnf update
# dnf groupinstall workstation
Ubuntu 18.04.x LTS
# apt update
[Install minimum GNOME desktop]
# apt install --no-install-recommends ubuntu-desktop
[Install full desktop with associated applications]
(Long process and too many extras installed)
# apt install ubuntu-desktop
[There are other alternative desktops and installations possible:]
[Generic Gnome desktop]
# apt install vanilla-gnome-desktop
[Mate]
# apt install ubuntu-mate-desktop
[Xfce]
# apt install xubuntu-desktop
[KDE]
# apt install kubuntu-desktop
[LightDM]
# apt install --no-install-recommends lightdm
Debian 9.x
# apt update
# apt install gnome-core
Debian 10.x
# apt update
# apt install gnome-core
All the above distros use systemd as their init system and you set the default run level with the same set of commands.
[Enable run level 5 by default]
systemctl set-default graphical.target
[Enable run level 3 by default]
systemctl set-default multi-user.target
Despite systemd you can still use init to start the graphical interface without having to reboot.
# init 5