In-app purchase Error while retrieving information from server [DF-AA-20]

DF-AA-20 means your app is not published in any way on the Play store. This is normally because either:

• you haven't published it yet. To test In-App Billing it must be pushed to at least alpha. See the testing IAB docs for more information
• your app or developer account has been banned/suspended for abuse
• you make some change to your package name/applicationId, so that in your apk it doesn't match the one on Play. Developers sometimes do this with build flavors

Oh, I had this exception when I tested buys in my product with different sku.
Check your sku!
Sku must be not CAPS! Only [a-z][0-9]!