iPhoto book

Which version of MacOS X do you have installed?

Are you running Photos 4.0 on Mojave and converted the book with a project extension? If yes, what happens, when you try to order from the project extension?

Or are you still running an older version of Photos? You can only order books from Photos, if you are using a project extension since September 30.

Or save your book as a PDF file from iPhoto or an older version of Photosand the upload the PDF to a printing service that can create books from the PDF, for example Presto Photos, see this link: Photo Book Printing, Color Book Printing, and Calendar Printing - PrestoPhoto

See: Old Toad: Post 9/30/2018 Printing Options for Photos

Also these support documents:

Use Project Extensions to order print products in Photos after September 30, 2018

Create and order print products in Photos with Project Extension

As you book is ready to be printed, consider to save it as a PDF and upload the PDF to presto Photos. Photo Book Printing, Color Book Printing, and Calendar Printing - PrestoPhoto

This page explains how to create a PDF of your iPhoto book:

https://support.apple.com/en-us/HT201504