It may depend on your script. There are scrips that look specifically for ebooks with a .prc extension and some with .azw and some with .mobi extensions.
If your script looks for .azw and the file you downloaded ends with .prc the script is not going to work.
That would be the first thing I would check. I would also check to make sure the plugin is active...
The way the script normally works is that it does is remove the DRM and adds the book to the library. The error you are getting tells me that either the script is not working right or it is not active