'Apply Cover Page.workflow' relies upon PDFtk server being installed. See http://www.pdflabs.com/tools/pdftk-server/.
For El Capitan you'll need https://www.pdflabs.com/tools/pdftk-the-pdf-toolkit/pdftk_server-2.02-mac_osx-10.11-setup.pkg. It's not on the pdflabs website yet as of 2011-02-05.