It seems it has something to do with the permissions of your admin directory. This mod will create a packing slip and then email it. The file is first created in your admin directory. After it emails it, the mod deletes this file. The script's packing list creation is being denied by your server. Try changing the permissions of your admin folder to 777. See if that fixes your problem. If it does, you know this is a permission problem. DO NOT leave your permissions as 777, as this is dangerous. Find a more secure setting that works for you. Hope that helps.
By the way, this is a questions for anyone who would be interested: I added an extra feature to this mod. It is extremely useful for me, but I am not sure if it would be valuable for others. It takes a lot of work to package up a mod for general consumption by Zen Cart users, so I wanted to see if anyone was interested in the changes. If not, I won't bother releasing it. Basically, this new version has a page where you can send POs for customers who did not buy through Zen Cart. So, for example, if you have a customer who places a phone order, buys through ebay, etc.... you can fill in a few details about them, choose the products from your database you want to send, and then it will email a PO with a Packing List attachment to a subcontractor of your choice. Nothing is recorded to the database for these customers, so tracking would still have to be handled by hand for these customers. This was extremely difficult to write, and I use it all the time, but I wasn't sure if there was the demand from anyone else for something like this. Let me know if you are interested. If people are, it will probably take a few weeks for me to get around to packaging it up, but I can do it. The only negative of this approach is that I didn't include all of the possible fields on this page, so fields like phone number are ignored. It would be quite cluttered to make a page with all these fields, and I don't use most of them anyway. It just uses the basics: Ship address, bill address, shipping method, PO Number, comments for PO and packing list, and all the products stuff: quantity, model, description, and attributes.