How to fix the "ERROR(OPF-030) - The unique-identifier ... was not found" error?

When you validate a EPUB 2.0 package with EpubCheck, you may see the following error:

C:\fyicenter> java -jar epubcheck.jar Error-Unique-ID-Not-Found-2.0.epub
 
Validating using EPUB version 2.0.1 rules.
ERROR(OPF-030): Error-Unique-ID-Not-Found-2.0.epub/package.opf(-1,-1): 
  The unique-identifier "MyID" was not found.

Check finished with errors
Messages: 0 fatals / 1 error / 0 warnings / 0 infos

EPUBCheck completed

Download Error-Unique-ID-Not-Found-2.0.epub, and unzip it. You see the "package.opf" file in the root directory.

<?xml version="1.0" encoding="UTF-8"?>
<!-- package.opf
  Copyright (c) 2018 FYIcenter.com 
-->
<package xmlns="http://www.idpf.org/2007/opf" version="2.0" 
  unique-identifier="MyID">
  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/">
    <dc:title>Hello</dc:title>
    <dc:identifier id="id">fyicenter.com.epub.hello</dc:identifier>
    <dc:language>en-US</dc:language>
  </metadata>

  <manifest>
    <item id="navigation" href="/navigation.xml" 
       media-type="application/x-dtbncx+xml"/>
    <item id="content" href="/content.xhtml" 
       media-type="application/xhtml+xml"/>
  </manifest>
  <spine toc="navigation">
    <itemref idref="content"/>
  </spine>  
</package>

The error is caused by the mis-matching values given in the (<package ... unique-identifier="MyID">) element and the (<dc:identifier id="id">) element. You can fix the error by changing id="id" to id="MyID".

⇒ ERROR(NCX-001) - NCX identifier Not Match

⇐ ERROR(PKG-005) - Mimetype Has Extra Field

⇑ Common Errors in Reported by EpubCheck

⇑⇑ EPUB Tutorials