Apple photos does not allow me to buy a photo book after I created it?

Apple no longer supports buying printed products in the same way that they did in the past. See the following: Use Project Extensions to order print products in Photos after September 30, 2018 - Apple Support

I can't recommend a specific extension -- hopefully someone else here will be able to do that.

I remember that service. Apple shut it down a while back. Your MacBook Air should be able to install Monterey. You may need to make a USB installer instead of downloading it straight to your MacBook Air.

How to create a bootable installer for macOS - Apple Support

Here are some links that should be compatible with High Sierra though. I do not have personal knowledge of these programs to say if you would be able to import your created album from the Photos app to them. My guess would be that you would have to create a new project specific with the app.

Mimeo Photos

Wix

GoodTimes

WhiteWall

Shutterfly

Did you use iPhoto or Photos to create the book?\

If iPhoto try the following;

Are you able to create a PDF file for that book by holding down the Option key and click on the Buy Book button.  That will give you a PDF file that is the same as was uploaded to Apple when they were still offering print services.

There's a 3rd party book provider which used to do the printing for Apple as a sub-contractor, Mimeo, that will take PDF files created by iPhoto and Photos and print and bind books and calendars. 

Also you can upload the PDF file to PrestoPhotos which can also print and bind books and calendars from iPhoto or Photos pdf files.  They've been doing it for some time now. 

Use the PrestoPhotos or Mimeo links above for direct access to the PDF file uploading and book/calendar ordering page for each site. 

Use the PrestoPhotos or Mimeo links above for direct access to the PDF file uploading and book/calendar ordering page for each site. 

Are you able to print the book to PDF (from Photos)? If you can you can also try uploading the pdf file to Mimeo for printing.

I keep Sierra on an external drive to boot into so I can run iPhoto just to create books. I like the themes that the last version of iPhoto offered and have gotten a couple of books that way. The quality was like it was when Apple was offering books.

The following by a very knowledgeable user here may also help: Post 9/30/2018 Printing Options for Photos - Apple Community