I looked rest of the code to check to look it was perfect, no missed by the followed the StoreKit tutorials by Apple Inc, and YouTube channel, too. I believed I was very closer to completed IAP! somehow, I noticed I got three codes to show the gray error that shows up after the app has frozen while it appeared sandbox account UIAlertController to purchase the products. One Code Results: Error: Enqueued from com.apple.main-thread (Thread 1) func purchase(product: IAPProduct) { 				 				guard let productToPurchase = products.filter({ 																												 						$0.productIdentifier == product.rawValue 						 				}).first else { 					 						return 						 				} 			 let payment = SKPayment(product: productToPurchase) 				/* Code Crashed 				paymentQueue.add(payment) 				*/ 		} I built UITableView to show an Episode with the Purchase version. The error was the same as the previous code. 		if indexPath.row == 3 { 												 				 /* code crashed 				 IAPService.shared.purchase(product: .epsiode4non_consumable) 				 */ 		} Now This is a final code that has shown one word by the error that says; "Thread" without mentioning any what is the issues. extension IAPService: SKPaymentTransactionObserver { 		func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) { 				for transaction in transactions { 						 						print(transaction.transactionState.status(), transaction.payment.productIdentifier) 						 						switch transaction.transactionState { 						 						case .purchased: 								 							 SKPaymentQueue.default().finishTransaction(transaction) 							 SKPaymentQueue.default().remove(self) 								print("Successful Purchased!") 								queue.finishTransaction(transaction) 								 						case .failed: 								 								SKPaymentQueue.default().finishTransaction(transaction) 								SKPaymentQueue.default().remove(self) 								print("Purchase is Failed.") 								queue.finishTransaction(transaction) 								 						case .restored: 								 								SKPaymentQueue.default().finishTransaction(transaction) 								SKPaymentQueue.default().remove(self) 								print("Purchase Restore is Successful.") 								queue.finishTransaction(transaction) 								 						case .deferred: 								 								SKPaymentQueue.default().finishTransaction(transaction) 								SKPaymentQueue.default().remove(self) 								print("Purchase is Deferred.") 								queue.finishTransaction(transaction) 								 								 								/* code crashed 							 default: queue.finishTransaction(transaction) 								*/ 						} 				} 		 } 	} I have no idea what is thread stands for? Someone can give me some tips about Thread for IAP issues?