Yes, you can!

When you checkout your purchase, choose the "Checkout with Credit Card" option, which will let you pay using the Stripe payments service, which covers payments from Turkey:

For context: Leanpub used to only use PayPal to process payments, and PayPal doesn't cover payments from Turkey. There was a period of time when customers from Turkey could, in some cases, make credit card purchases anyway, but we made an update that inadvertently prevented them from doing so for a few months, until we added support for Stripe. If you encountered this problem, we hope you'll please accept our sincere apologies for that!