Synaptic doesn't launch as root [duplicate]

When I'm trying to launch Synaptic from Application Manager it asks me for password, I'm writing it ( I doube-checked though) and then the software close.

When I'm trying to launch it with sudo it doesn't work, and everytime when I'm trying to log in with su in terminal, I get Authentication failure and I'm writing the correct password. What's wrong??

Output for sudo -H synaptic

No protocol specified
Failed to connect to Mir: Failed to connect to server socket: No such file or directory
Unable to init server: Could not connect: Connection refused

(synaptic:32380): Gtk-WARNING **: cannot open display: :0