The generated Table of contents is incorrect with a "start" tag

Every .epub file has to have a table of contents (toc.ncx) regardless its topic, but that doesn't mean that it will necessarily appear in the book. If you only have one entry in your table of contents (the beginning of your book if it doesn't contain chapters), that makes your .epub valid.

The problem with your table of contents is that it doesn't contain any entries: it only contains an incorrect 'cover' or 'start' tag. The editor you used probably knew that it had to create a table of contents but it created an incorrect one.

Open the book with Sigil. Locate the toc.ncx:

Change the content so that it reflects this example:

The 'Book title' should be the title of your book, and the content src should point to the file where your book starts.​