Why: Ordering is no longer available for this version of Photos.
If you are creating a new photo album in Apple Photos app, you will not be able to order a hard print through Apple anymore. Apple officially discontinued the built-in print service for Photos on September 30, 2018.
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.
The Photos app will transition to Photos Project Extensions entirely in later version of MacOS. Apple suggests to install Project Extensions, which will require you to update to macOS High Sierra if you are not on it already.