We use PayPal Website Payments Pro to process our credit card form, and they may be blocking your transaction as part of a fraud prevention strategy. It may also be that you are trying to make a purchase from a country PayPal does not support. Here is a list of countries supported by PayPal: https://www.paypal.com/us/webapps/mpp/country-worldwide.
You could try checking out using the PayPal option we provide. This process is separate from PayPal Website Payments Pro, and sometimes payments work using PayPal directly, even if they don't work when you use our credit card form.
You could also try paying again later, or with another card. Unfortunately, we are not provided with any more specific information that could help us determine the exact cause of this issue.
There is also a possibility that your financial institution blocked the payment in order to protect you from potential fraud. In that case, the best thing to do would be to contact your financial institution and confirm that your purchases from us are legitimate purchases by you.
If you are unable to purchase a book because PayPal does not support purchases in your country, we suggest that you go to the web page for the book you are trying to buy, and you scroll down until you see the "Email the Author(s)" button. You can use that feature to explain the situation to the author, and ask for a free coupon, since you are unable to purchase the book.