Never upgrade to later version of Kindle App, stick with 1.19.

The post goes on to suggest how one can workaround this issue. These books will appear in a subdirectory of the “My Kindle Content” folder, will import into calibre as “AZW” format, and will fail to convert with the error: “KFXError: This is an Amazon KFX book. The direct consequence of this action is that Calibre ( with DeDRM plugin) that many people love to use, can no longer strip the DRM off from Amazon ebooks, and can no longer convert the Kindle ebooks to MOBI or EPUB format. Starting from Kindle App 1.19 onwards, Amazon changes the format of the downloaded Kindle ebooks to KFX format. Use Calibre to remove DRM only when you are certain that you have the rights to read the books anywhere and in however formats that you want. Although it is not wrong to tell people how to remove the DRM( ie: this post is not wrong), we are not condoning piracy: we do not recommend you strip off the DRM and share/sell the books in violation of copyrights laws.

Convert azw to mobi remove drm