Enter the hostname of the machine running the NPrinting Scheduler service (NPrinting Server). See point 6 in the online help: https://help.qlik.com/en-US/nprinting/February2018/Content/DeployingQVNprinting/Adding-a-Qlik-NPrinting-Engine.htm
To determine the computer name, open the System properties page and use the Computer name parameter, without the domain name. You cannot use the IP address.
HTH - Daniel.