I can't enable lock screen mode in my laptop and also in my PC. I went to system setting > screen > lock
Ctrl+Alt+L :
Nothing happens!
the shortcut is there but not works !

sudo apt-get --reinstall install gnome-screensaver :

Reading package lists... Done

Building dependency tree       

Reading state information... Done

You might want to run 'apt-get -f install' to correct these:

The following packages have unmet dependencies:

 dassault-systemes-draftsight:i386 : Depends: sendmail:i386 but it is not installable

                                     PreDepends: libdirectfb-extra:i386 (>= 1.2.7-2) but it is not going to be installed

                                     PreDepends: libxcb-render-util0:i386 (>= 0.3.6-1) but it is not installable

E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).