We've seen this error when a file is referenced from a network share. An example would be a setup.iss file.
PDQ Deploy copies all installation files to a target computer before starting an installation. If you have additional files that are required as part of the install, simply check the "Include Entire Directory" checkbox in your PDQ Package. This will copy all files and directories at or below the install file referenced.
In the parameters field you can then specify the file (.iss or other file) without using the path or you can reference the path by using .\filename.iss