I'm been having this error whilst trying to publish my book to Blurb. I've tested the issue by selecting PDF output, but the same issue persists.
I've painfully gone through all 200+ pages of my book replacing the fonts of what I thought was causing the issue, but unfortunately the issue persisted.
I'm at a loss what to do now, as countless months has been spent producing this book in Lightroom but I'm now unable to publish it.
I'm using Lightroom CC 2015.10.1 [ 1117303 ]
Mac OS 10
VERSION 10.12 
Could you try changing the font and see if the problem persists?
1.Select all the text cells - Edit > Select all text cells
2.Select a different font in the Type palette.
You may want to duplicate your book first (collections panel >right click on the book and select duplicate book).