Preflight is only showing you problems.
To correct problems you have to go to the document, in many cases the preflight has a number where it helps you to jump, but with wrong page numbers you have to go to the Page Panel and add some pages or delete some.
For books, a sheet of paper is folded in half, this gives you four pages when do so. Try it with a piece of paper.
If your book is to be printed, then it needs to be in multiples of 4, that is it has to be say, 48 pages in the document, or 52 pages, but 50 pages would not do, as it's 2 pages short of what's needed.
Usually it's not an issue for professional printers, as they would add in 2 blank pages at the start or the end of the book.
But you have control over this now - so make sure you book page count is in multiples of 4.