The error message you get is a bit confusing. If you want to order a book directly from Photos, you now need to use project extensions to order from third party providers. And these extensions are only available in Photos 3.0 or 4.0.
In Photos 4.0 the tools to create an Apple-calendar or book project or cards have been removed completely.
If you upgrade to Mojave or High Sierra you can install third-party project extensions in Photos. The project extensions may be able to convert your project to a new format and you can order the project from a third-party Print service. But I am not sure, if this already working for Calendars, see:
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
Presto can print the book like it has been formatted in Photos or iPhoto.
If you upgrade to Photos 4.0 and use the existing extensions to convert your book, there may be differences.