That error occurs when you don't have write access tithe directory where you're trying to save the file. I'd recommend coding the path to the local desktop. It'll work on either platform (other than filewin or filemac) and users have read/write access to their desktop.
I had the option: "specify options checked" in my append pdf script.
This option has to be unchecked as the options are defined at the creation of the pdf and can't be modified later.
This solved my problem.