There might be a number of reasons and we will try to add more of them as they are brought to our attention. Mostly, they are to do with some inconsistencies in your book file. Here is an example:
- Null ID links: If you created your book from a WORD document, you might face null ID links. Here is such a link:
- <a herf= "https://www.w3.org/TR/WCAG21" id: "" target="_blank" rel="no opener no referrer">
- whereas it needs to be edited to "https://www.w3.org/TR/WCAG21"