We're sorry to hear you're having an issue with the book when you open it in your PocketBook reader. Thanks also for sharing the image, that's really helpful.
There are lots of ebook readers out there, and all Leanpub books don't always work on all readers, unfortunately.
Here is an article about the apps and devices we do support:
Please note we make it easy to get a refund within 60 days of purchase:
Our apologies for not having a better answer for you!