No Title Bar button to close window Ubuntu 18.04
Your window-control button layout doesn't look right. Run the following command in Terminal to reset it
gsettings reset org.gnome.desktop.wm.preferences button-layout
This should fix the problem. If it doesn't work, then set a suitable button layout manually. For example run the following command
gsettings set org.gnome.desktop.wm.preferences button-layout :minimize,maximize,close
to get the minimize, maximize, close buttons (in this order) at the right end of the title-bar.
Alternatively to get the close, minimize, maximize buttons (in this order) at the left end of the title-bar, run
gsettings set org.gnome.desktop.wm.preferences button-layout close,minimize,maximize:
and so on.