salve,

HPLIP cannot detect printers in your network.
This may be due to existing firewall settings blocking the required ports. When you are in a trusted network environment, you may open the ports for network services like mdns and slp in the firewall. For detailed steps follow the link. http://hplipopensource.com/node/374

Il messaggio dice che le porte della stampante possono essere bloccate dal firewall.
Controlla le impostazioni.