Are you trying to order a printed book from Apple?
Apple is no longer offering any print services at all. The last orders had to be placed by September 30, 2018.
The Photos.app is prompting you to upgrade to a new system version, because only newer system versions allow you to use third-party project extensions to create books and have them printed by third-party services. Upgrading to macOS High Sierra will not necessarily enable you to get your existing book printed. You would need to recreate large parts of the book using project extension and lose your work. None of my existing books had been migrated well, when I upgraded to High Sierra.
Since your book is already finished, save the book as a PDF file.
The Mimeo project extension is supposed to be able to convert your book, but it did not work well on High Sierra. It will be safer to upload the PDF file.
Create the PDF as described here: Check and review a project before you order using Photos on Mac - Apple Support