SKErrorDomain Code=0 "Cannot connect to iTunes Store" in IAP iOS Objective c

when i tested IAP for my app, it throws following error

Error Domain=SKErrorDomain Code=0 "Cannot connect to iTunes Store" 
UserInfo={NSLocalizedDescription=Cannot connect to iTunes Store}

i have tried many links related to this problem but nothing helped. Moreover the "Product Identifier" are also correct and i also created new sandbox user, but no output. Any help would be great.

What does Skerrordomain 0 mean?

Your test user has become invalidated. This can happen if you accidentally log into the App Store with your test user. When this happens, you'll get error 0 after entering your password to confirm your payment.

What is Skerrordomain?

The error domain name for StoreKit errors.

Looks like Apple's servers are down for Sandbox environment. Live purchases are working just fine!