We're sorry to hear you're encountering this error. It's rare, but it has been reported before, which is why we've written this article. We hope it helps!
First, we recommend you try making your purchase in a different browser, or on a different device.
Second, it actually might be a temporary issue, and it will work if you "try again later".
Third, it's possible that there really is an issue with PayPal, or even that your financial institution is blocking the transaction, in order to protect you from possible fraud (for example, if you are using a credit card to make a purchase).
In that case, you should contact PayPal to see if they can help. In our experience, their customer support is usually very good.
If you do need to contact PayPal or your financial institution, please note that the following business information about Leanpub might be helpful: