A book that is offered for free cannot be made available for pre-order. Your free book will be offered on Apple Books on the scheduled release date. You can update the release date or pricing for the book in My Books on iTunes Connect.
Generally this happens when the series metadata submitted wasn’t consistent with the series metadata on file for the other books. Any differences in the series name including spacing, punctuation, or additional words will result in the book not properly linking to a series.
You can confirm what series information Apple Books has by requesting a Book Catalog Report from My Books.