Sign-In Required Popup in-App purchase issue?

As far as I know, this happens in two cases.

1. There is a pending transaction.If some purchase transaction haven't finished yet, iOS will try to proceed that transaction automatically.

2. Some of auto-renewable subscription should be renewed.iOS also automatically start a purchase transaction for auto-renewable transaction, if auto-renew setting is on and subscription is expired.

This is triggered when transaction observer is added to payment queue

[[SKPaymentQueue defaultQueue] addTransactionObserver: yourTransactionObserver];

In any case of them, you can put break point in your transaction observer

- (void)paymentQueue:(SKPaymentQueue *)queue updatedTransactions:(NSArray *)transactions

and grasp which transaction is processing.

Basically, SKPaymentQueue has no function to cancel ongoing transaction by the app, so you owe to proceed that transaction.

As you mentioned , 'Now, I uninstalled apps from my device, also clear credential from iTunes & App Store. and again installed.', you need to login with apple account in itunes on device to remove this pop up.

This happens sometime in sandbox environment not always. Once your app is live and in app purchases are approved by Apple, everything will work fine automatically.

However, your in-app purchase code flow should be correct. iOS automatically ask for login options if you have not logged in yet, otherwise it will show popup for entering password only and username/email will be auto filled.