My little Ubuntu machine wouldn’t let me shut down. I had to click on “Dash Home” and search for the term “shut down” and then it would do it, but there was no button for me to click to get it to shut down without searching. Very annoying. When I looked up this problem online, I realized that there should have been a little “gear icon” at the top right of the screen. There wasn’t. I don’t know why, but this seems to be a problem with the theme called “Ambiance”. You can fix this problem by changing your them to “Radiance” instead.
Settings –> Appearance –> Theme –> Change from Ambiance to Radiance
This change also added extra icons to the top menu and provided actual pictures for the “Settings” and “Trash” buttons on the side menu. With the Ambiance theme, these were all blank.
Hope this helps!