Hey, @davidp45932171. Welcome to the Lightroom Classic community. I'll help you figure this out.
What version of Lightroom & the operating system are you working on?
Test these suggestions:
- Ensure the Book title is only alphabet & does not include unique characters & numbers.
- Your book does not have missing images.
- Try to sign out & sign in again to the Blurb.
As a temporary workaround, manually change the book type from under Book Settings to PDF & export the book as PDF & send it over to the Blurb with your preferences.
Let me know how it goes. Thanks!