Nexus 7 - trouble in setting up Google wallet for app purchase

I'm using the nexus 7 with jellybean 4.2.. I was trying to buy an app but it's not accepting my debit card.. I tried 2 ways
1. Online play store - when I fill all the debit card details and submit it says invalid card.. I tried 3 different debit cards but of no use
2. I tried purchasing an app through play store app on my tablet and after filling all details it gave me an error "Error retrieving information from server. [RPC:S-7:AEC-0] "
Can someone help me on this?
How come you guys are purchasing an app from play store?