We recently finished a client’s book that’s 91 pages long. I downloaded the KDP cover template. Because those come in ten-page increments, I was given the template for a 100-page book. The template includes space for text on the spine, so as usual I put the book’s title and author’s name there.
When I submitted the book for publication, it was rejected because a book has to have at least 100 pages in order for the cover to have text on the spine. Fair enough, but KDP apparently doesn’t have an appropriate template for a book of 91-99 pages in length. Instead, you get the 100-page template with space for text on the spine, which causes an error.
So be aware of this problem. If your book is under 100 pages but the template shows a space for text on the spine, don’t put anything there.