Occasionally customers using our credit card form report seeing the error "This transaction cannot be processed."
This can happen for a number of reasons. We suggest you try checking out with the PayPal option, instead of our credit card form, to see if that works. You can even use a credit card on the PayPal site; you don't need to already have a PayPal account.
(And yes, sometimes a credit card which does not work on the Leanpub site does work on the PayPal site, even though we use PayPal for our credit card processing. These are some of the joys of internet commerce.)
If that does not work, we also suggest using another credit card and/or a different browser or device, or just trying again later.
Sometimes this error occurs when a customer's bank is blocking the transaction to protect the customer from fraud. Here are some instructions for what to do in that case: http://help.leanpub.com/reader-help/my-purchase-was-blocked-why-is-this-happening-and-what-can-i-do.
Occasionally this error is also due to script-blocking Chrome plugins or something similar. If you turn off any browser plugins you are running, then the purchase might work.
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.