Which version of Lightroom are you using?
Try disabling your Computer Firewall,Antivirus
This happens if Lightroom is unable to reach Blurb servers when something is preventing it to connect.
Please refer Issues when uploading book to Blurb from Lightroom 5
Let us know if that helps.
Do try again over the course of a few hours as their servers could be done or there could be issues on their end. I've seen this in the past.