[theducks]

Originally Posted by AllenW

I am using an EPUB file created by my ghostwriter and it now has E21027 errors when I upload to KDP/Amazon. I wonder if my writer used CALIBRE to create the EPUB file. I have no resolution yet.

Calibre leaves its fingerprints throughout the files. All you need to do is unpack the EPUB and look in the code for the word calibre with a simple text (only) editor (like Notepad.exe)

Just do this on a COPY
change the file extension to ZIP, then unzip. Look in Content.opf

<?xml version='1.0' encoding='utf-8'?>
<package xmlns="http://www.idpf.org/2007/opf" version="2.0" unique-identifier="uuid_id">
  <metadata xmlns:opf="http://www.idpf.org/2007/opf" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:dcterms="http://purl.org/dc/terms/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:calibre="http://calibre.kovidgoyal.net/2009/metadata">
    <dc:title>Forgotten: The Box Set</dc:title>
    <dc:creator opf:file-as="Forbes, M. R." opf:role="aut">M. R. Forbes</dc:creator>
    <dc:language>en</dc:language>
    <dc:contributor opf:role="bkp">calibre (7.16.0) [https://calibre-ebook.com]</dc:contributor>
    <dc:description>&lt;div&gt;
&lt;p&gt;Contains :&lt;/p&gt;
&lt;p&gt;Fogotten (3 books)&lt;/p&gt;
&lt;p&gt;Forgotten Earth (4 books)&lt;/p&gt;&lt;/div&gt;</dc:description>
    <dc:identifier opf:scheme="MOBI-ASIN">B0DBFX31HF</dc:identifier>
    <dc:identifier id="uuid_id" opf:scheme="uuid">urn:uuid:fcdaa910-d2c0-4bde-beaf-be1d807d076a</dc:identifier>
    <dc:identifier opf:scheme="calibre">urn:uuid:fcdaa910-d2c0-4bde-beaf-be1d807d076a</dc:identifier>

and many mor, depending on what calibre was used to do.