You will get an error when trying to convert DRM-protected ebook with
Calibre.
What version of
Kindle for PC or
Kindle for Mac do you have? Or have you downloaded your ebook using
Kindle reader?
What version of Calibre DRM removal plug-in do you use?
When you get an error using a DRM removal plug-in, it is always a good idea to try to use standalone DRM removal tools instead. The latter tools always give a better error description.