StoreKit testing in XCode

I'm trying to use StoreKit testing in XCode, but not work. It go through all steps in iPhone side on the purchase workflow e.g. prompt that telling me the purchase won't charge me as it is for testing only (purchase a consumable item through our app) but it fail finally. The problem is we follow Apple workflow: iphone post the receipt to our server and our server will send the receipt to https://sandbox.itunes.apple.com/verifyReceipt for validation, however, we receives a non-zero status value from apple (21002). Any idea what we missing in setup ?