That message indicates that the Kindle Previewer is failing to convert your book. Some suggestions:
Make sure that your system is compatible with the Kindle Previewer. Recent versions require 64-bit Windows version 8.1 or higher.
Try converting
this edition of Frankenstein. If that works then the problem is probably due to the book(s) you are attempting to convert.
If you are using anti-virus software other than that provided with Windows try disabling it before conversion.
Try uninstalling and re-installing the Kindle Previewer.