This error usually comes up when a customer's bank is blocking the transaction for some reason. Occasionally it's also due to script-blocking Chrome plugins or something similar.
If possible we would suggest using another credit card and/or a different browser or device, or just trying again later.
We're sorry for not having a perfect response for you on this matter. Payment issues like this sometimes involve variables over which we have no control.
You may also want to read this article if you're looking for more information: http://help.leanpub.com/reader-help/my-purchase-was-blocked-why-is-this-happening-and-what-can-i-do.