print an iphoto project

Which version of MacOS X do you have installed?

You need macOS 10.13 High Sierra or macOS 10.14 Mojave to use project extensions. On older systems export a PDF file of the project and have this PDF printed, see: Ordering Books and Calendars on a Mac tha… - Apple Community

On a Mac with a newer system you can create a converted project.

That is described here: Create and order print products in Photos with Project Extensions - Apple Support

Convert existing projects

You can update or continue to work on your existing Photos projects by converting them using third-party extensions:

1. 1. In the Photos sidebar under Projects, select the project that you want to convert.
   2. In the Photos menu bar, choose File > Create, and then choose the Project Extension that you want to use. The extension shows options for the project, then converts the project into its own format.
   3. Use the Project Extension's interface within Photos to edit your project and place your order for print products. Click Done in the upper-right corner of Photos when you're finished. The converted project appears in the sidebar along with your original project.

After you convert your projects, you can edit them further and place orders using the extension's interface within Photos.

"I do have the right version of Sierra. "

You need High Sierra at least not Sierra.

It is natural that the reviews are currently not good, since the extensions are all very new, and there are teething problems.

The easiest way out would really be to save your project as a PDF file and have this printed. There is no guarantee, that the converted project will be identical to the book or calendar that you have now. there ma bye differences.

To get an idea which extension to chose, have a look at OT's user tip:

Post 9/30/2018 Printing Options for Photos - Apple Community

Mimeo and Motif can convert projects. Mimeo used to print for Apple and the books are very similar: