It looks like you are not using the standard MyBookTable Buy Buttons for your "Buy Now" button on that book, so we are unable to help with this issue. I would recommend contacting the support for whichever plugin or website you used to create this button.
If you're having trouble using MyBookTable Buy Buttons, you should try checking out the MyBookTable > Help page on your WordPress Admin Menu. There is a "How to add buy buttons" video there that should help you out. If you have further questions after watching that video, please let us know.
Regards,~Tim