We use PayPal Website Payments Pro to process our credit card form, so if you're in a country where PayPal doesn't operate, you won't be able to purchase a Leanpub book.

Here is a link to a page that shows the countries where PayPal operates: https://www.paypal.com/ca/webapps/mpp/country-worldwide.
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If you are unable to purchase a book because you're in a country that doesn't support PayPal, you can try going to the book's landing page on Leanpub and clicking on the "Email the Author(s)" link you'll find if you scroll down (unless the feature has been disabled by the author or authors).

You can use this feature to email the book's author(s) and ask for a free coupon, which the book's author(s) may be willing to provide if you explain your situation.