Hi Erik Garcia,
This error occurs when a variable is empty or null. Likely something is missing when trying to save the pdf.
I would double check the save directory, title or any other variables on the pdf export tool.
Otherwise if that does not work, best way to troubleshoot is to rule out what could be causing the errorfrom within your template. save a copy and start to remove items, try run the tool after removing each item, hopefully you will find the troublesome template item that is causing the error and can go from there.
Cheers,
Cam