Update manager does not show version 18.04 as upgradable, and from the console I write update-manager -d and it answers "Gtk-WARNING **: cannot open display: :0 No protocol specified Unable to init server: Could not connect: Connection refused". I use version 17.10
What can I do?
I solved the problem. Thanks, Norbert, Karel and K7AAY. In fact I did everything you told me, and one of your solutions worked, though I am not sure which one did. My problem was related to the graphic system, gtk, so the upgrade worked in console mode.
I'm grateful to you all, because your answers made me think! :-)