This is happening for my testers too. It clearly can connect to the App Store, because it downloads the list of available IAP and prices. I think it's failing in the authentication process when they try to buy an item. I've had some occasional success with some testers, but 80% of them fail with that "Cannot connect" error.If it's any use to you, I created a single "Consumable" IAP called "Tip the Developer." Because it's consumable, your testers can keep testing IAP over and over again, which means you don't need extra sandbox testers, or any (but this one) fake IAP items to test with.