Trying to order a book created on iPhotos and get message that says to upgrade - how?

Apple stopped the service for print products after September 30. Since then you can no ,longer order print products from Apple.

You can now order books and calendars using the project extensions in Photos 4.0 on Mojave. This requires a system upgrade. iPhoto cannot be updated any longer at all.

See:

Here is a recent user tip by OT: : Old Toad: Post 9/30/2018 Printing Options for Photos

See these support documents: Create and order print products in Photos with Project Extensions - Apple Support

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

Since your book is already finished, you could save it as a PDF file and send it to a different print service, for example Presto Photos. Presto Photos can print iPhoto PDFs and Photos PDFs.

See this link: Photo Book Printing, Color Book Printing, and Calendar Printing - PrestoPhoto

To create a PDF file of your book see: Preview a print project in Photos or iPhoto to avoid issues - Apple Support

If you're able to do a print to PDF with the book in your version of iPhoto you can order the book from PrestoPhotos. They can print and bind books created from the PDF file that iPhoto creates.