Step 15 book generation errors occur when the PDF version of the book fails to generate.
Sometimes this is caused by a book cover image that is too big. If you recently updated your book cover image, that may be the issue.
You can update your book cover image by going here in the Author menu:
Author > Books > [Book Title] > Settings > Upload Book Cover
Sometimes Step 15 errors are also caused if an author has been adding characters we don't accept to e.g. the "Tweet this book!" section here:
Author > Books > [Book Title] > Community > Social Media
If you don't think any of these suggestions are causing the problem, please email the Leanpub team at firstname.lastname@example.org.