diametrica@diametrica-System-Name:~$ sudo apt-get install gnome-screenshot
[sudo] password for diametrica: 
Lettura elenco dei pacchetti... Fatto
Generazione albero delle dipendenze       
Lettura informazioni sullo stato... Fatto
Package gnome-screenshot is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'gnome-screenshot' has no installation candidate
diametrica@diametrica-System-Name:~$