ProcessViewerPageURL is only generated in the Event Configuration tab of a Dialog Workflow component and can only be used there. You can create your own url to view the ticket by using a merge text and creating the following string:
http://servername/ProcessManager/Reports/ProcessReportDetails1.aspx?ReportProcessID=Global.ReportProcess
The Global.ReportProcess is from the Global variable and then in the email you would change the drop-down to a Variable Web Link and drag that merge text over to the email body.