I am sorry, but there is no easy fix for this problem. Conversion to KFX relies on Amazon's Kindle Previewer 3 beta, which has serious bugs and you have found one.
You may be able to make it work for your book, but it is probably going to require a lot of effort on your part. You will need to figure out what part of the book is triggering the error and change it to avoid the problem. To do this you will need to learn a lot about how e-books work internally and how to edit them.
This will take a trial and error process of removing or changing parts to the book to narrow down what specific item is triggering the error. You will then need to find a change to the book that retains the content and formatting without causing the error. The result may not be worth your effort.
You may want to investigate the
Hyphenate This! calibre plugin as an alternative to KFX conversion.
In the future it would be best to ask questions about conversion to KFX in the
KFX Output Plugin thread.