It is no longer possible to order photo books from Apple. The last orders had to be filed by September 30.
: Use Project Extensions to order print products in Photos after September 30, 2018 - Apple Support
To give users access to a broader variety of print services, macOS High Sierra introduced Photos Project Extensions which integrate directly with the Photos app. In macOS Mojave, Photos will transition entirely to Photos Project Extensions. Users will be able to print and update their existing projects using these new services, and will gain access to a broader variety of options.
For final purchases using the existing built-in service, place your orders by September 30, 2018.
Now we have to order our books from third-party providers. OT compiled a list of the existing project extensions: Old Toad: Post 9/30/2018 Printing Options for Photos
See also: Create and order print products in Photos with Project Extension
To get your existing book printed, you can try to convert your existing book with the Mimeo Project extension, or save the book as a PDF and order the PDF to be printed by Presto Photos.
The warning message you received to upgrade the system is referring to the fact, that the project extensions to create books and calendars are only supported in Photos 3.0 on High Sierra and Photos 4.0 on Mojave. Project extensions are currently the only way to create print products and order them for printing, and only from third party print services.