It has been confirmed with Qlik that the OnDemand button only supports Windows NTLM authentication, that is when a user logs in to Qlik Sense via Windows, the OnDemand button then logs the user into NPrinting via Windows and the reports get generated immediately.
However, when a user is logged in to Qlik Sense via SAML authentication in Qlik Sense and NPrinting, The OnDemand button does not automatically authenticate the user into NPrinting and the report is not generated. This is where the problem is and this feature / process does not exist in the OnDemand button functionality yet.