How to solve the Enqueued and Thread issues thought the In App Purchase?

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)

Code Block  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.

Code Block     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.

Code Block  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?