Sounds like the merge is failing, but the error message may not be accurate.
Did the Demo ever work on your server, if so, what was changed from then to now? If the plugin has never worked, we may need to take a look at your installation to be sure there is not a server-side issue. If possible, please complete this form to allow us to check your installation.
If not possible, try testing by removing all merge fields from the Field Map Layout except one name field. If the merge still fails, email support AT formidablepro2pdf DOT com the screenshots of the Export Tab (with the first three dropdowns selected to export a pdf), Activated Forms tab, and the Under the Hood tab.