3D FlipBook - Category Shortcode Displays "No PDF URL provided."

When I paste the category shortcode into a page, i get the message "No PDF URL Provided". There are no issues when I paste the shortcode for an individual PDF, only when trying to display pdfs from a category. Anyone have any ideas why that may be the case? I want to upgrade to Pro but want to see if anyone else has this issue/resolution before I pull the trigger.

Thanks!