Your ePub file unfortunately has one or more errors. Here you can find the most common HTML errors in an ePUB file:

1. Errors were found in an image file:
The width or height of the cover is too small. The cover in the ePUB file must have at least 625 x 1000 pixels and should be in RGB color space. The cover must not have more than 4,000,000 pixels.

2. E_PACK_ERROR Invalid UUID string:
The content OPF contains an unreadable ID in the <dc:identifier> field. Usually structured according to this pattern: 'urn:uuid:995-d29-derjw31-215aaa9' Simply replace 'urn:uuid:...' with the ISBN number of the EPUB.

3. Errors were found in the OPF file:
There is no language(s) stored in the metadata. The tag <dc:language> is missing in the metadata area of the OPF file.

4. Errors were found in some HTML files:
The file name contains special characters/spaces. An HTML file contained in the EPUB contains spaces or similar special characters in the name. The files must therefore be renamed. (IMPORTANT: If the name of an HTML file is changed, the TOC link and the OPF manifest listing must be adjusted.)

5. Errors were found in the EPUB structure:
The file is not a valid ZIP file. The EPUB could not be validated. The reason can be incorrect programming within the META-INF folder, i.e. in the files "content.opf", "mimetype", etc. For example, an umlaut may be entered in the wrong place or a tag may be entered incorrectly. This can happen if you create EPUBs with programs like Calibre that use tags other than those specified in the IDPF guidelines, or if you work manually in the OPF file. Make sure that the mimetype says "application/EPUB+zip" and that this file was compressed first.

6. Errors were found in the NCX-file:
The NCX file contains less than three entries. At least cover, imprint and title must be listed. The EPUB must contain a complete and, if possible, differentiated table of contents: In the corresponding file TOC.ncx at least the main sections (cover, imprint and title) and the chapters of the content part must be linked as navigation points. If the table of contents contains too few navigation points, some shops consider it not user-friendly and therefore do not accept it.

7. HTML Error | Description is too long
The description in the metadata of your ePUB is too long (<dc:description>). The maximum number of characters is 250.

8. an error has occurred in your PDF file
Important information is missing in the metadata: Title and/or author.