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Step 15 book generation errors occur when the PDF version of the book fails to generate.
If this is the first time you're encountering this error, its cause may be due to a change you have made recently. Undoing your recent changes and then trying again to see if you can narrow down the issue is a good next step.
You can also download the latest working version of your manuscript by following these instructions and then comparing your current manuscript to your last working manuscript, to look for any differences that may be causing the problem.
If this doesn't help, please read our Leanpub Author Support Guidelines, for some information about what to do next: