В проблеме покупки приложения с SKProduct() в Swift

Ваша функция getProduct не делает ничего, кроме вывода пустого массива (products), фильтрации этого пустого массива и извлечения первого элемента (которого не существует).
Предлагаю использовать такой помощник (я скопировал его из где-то, не помню где):

import Foundation
import StoreKit

class IAPHelper: NSObject, SKProductsRequestDelegate, SKPaymentTransactionObserver {

  typealias RequestProductsCompletionHandler = (_ success:Bool, _ products:[SKProduct]?) -> ()
  var completionHandler: RequestProductsCompletionHandler?
  var productsRequest: SKProductsRequest?
  var productIdentifiers: Set<String>
  var purchasedProductIdentifiers: Set<String>?

  // MARK: - Init  

  init(identifiers: Set<String>) {

    // Store product identifiers
    productIdentifiers = identifiers;

    // Check for previously purchased products
    purchasedProductIdentifiers = Set()
    for productIdentifier in productIdentifiers {
      let productPurchased = UserDefaults.standard.bool(forKey: productIdentifier)
      if productPurchased {
        purchasedProductIdentifiers?.insert(productIdentifier)
                print("Previously purchased: \(productIdentifier)");
      } else {
                print("Not purchased: \(productIdentifier)");
      }
    }
    super.init() // must be called after subclass init, but before "self" is used
    SKPaymentQueue.default().add(self)
  } // init

  func requestProductsWithCompletionHandler(_ completionHandler: @escaping RequestProductsCompletionHandler) {
    self.completionHandler = completionHandler

    productsRequest = SKProductsRequest.init(productIdentifiers: productIdentifiers)
    productsRequest!.delegate = self
    productsRequest!.start()
  }

  // MARK: - Products request delegate

  func productsRequest(_ request: SKProductsRequest, didReceive response: SKProductsResponse) {
    productsRequest = nil

    let skProducts = response.products
        for skProduct in skProducts {
            print("Found product: \(skProduct.productIdentifier), \(skProduct.localizedTitle), \(skProduct.price.floatValue)");
    }

    completionHandler!(true, skProducts)
    completionHandler = nil
  }

  func request(_ request: SKRequest, didFailWithError error: Error) {
        // NOTE: If this happens on the simulator, close the simulator window and re-run the app. This helps normally !!!
        print("Failed to load list of products. Error: \(error)")
    productsRequest = nil

    completionHandler!(false, nil)
    completionHandler = nil
  }

  // MARK: - Purchase

  func productPurchchased(_ productIdentifier:String) -> Bool {
    return purchasedProductIdentifiers!.contains(productIdentifier)
  }

  func buyProduct(_ product:SKProduct) {
        print("Buying \(product.productIdentifier)");

    let payment = SKPayment(product: product)
    SKPaymentQueue.default().add(payment)
  }

  func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {
    for transaction in transactions {
      switch transaction.transactionState {
      case SKPaymentTransactionState.purchased:
        self.completeTransaction(transaction)

      case SKPaymentTransactionState.failed:
        self.failedTransaction(transaction)

      case SKPaymentTransactionState.restored:
        self.restoreTransaction(transaction)

      default:
        print("Error: Unknown SKPaymentTransactionState")
      }
    }
  }

    func completeTransaction(_ transaction:SKPaymentTransaction) {
        print("completeTransaction...");

        self.provideContentForProductIdentifier(transaction.payment.productIdentifier)
        SKPaymentQueue.default().finishTransaction(transaction)
    }

    func restoreTransaction(_ transaction:SKPaymentTransaction) {
        print("restoreTransaction...");

        self.provideContentForProductIdentifier(transaction.original!.payment.productIdentifier)
        SKPaymentQueue.default().finishTransaction(transaction)
    }

    func failedTransaction(_ transaction:SKPaymentTransaction) {
        print("failedTransaction...");
        if let error = transaction.error as NSError?, error.code == SKError.paymentCancelled.rawValue {
                print("Transaction error:\(error.localizedDescription)");
        }

        SKPaymentQueue.default().finishTransaction(transaction)
    }

    func provideContentForProductIdentifier(_ productIdentifier:String) {

        purchasedProductIdentifiers?.insert(productIdentifier)
    UserDefaults.standard.set(true, forKey:productIdentifier)
    NotificationCenter.default.post(name: Notification.Name(rawValue: IAPHelperProductPurchasedNotification), object: productIdentifier, userInfo: nil)
  }

  func restoreCompletedTransactions() {
    SKPaymentQueue.default().restoreCompletedTransactions()
  }

}

Для получения товаров необходимо произвести SKProductsRequest и дождаться ответа в SKProductsRequestDelegate способах.

Создайте и запустите запрос:

let request = SKProductsRequest(productIdentifiers: ids)
request.delegate = self // self is delegate of SKProductsRequestDelegate, see below
request.start()

Где ids должно быть Set<String> ваших идентификаторов продуктов.

Соответствовать SKProductsRequestDelegate:

extension MyClass: SKProductsRequestDelegate {

    internal func productsRequest(_ request: SKProductsRequest, didReceive response: SKProductsResponse) {
        print(response.products) // here is your [SKProduct] array
        print(response.invalidProductIdentifiers) // if you put wrong ids they would be here
    } 

    internal func request(_ request: SKRequest, didFailWithError error: Error) {
        print(error) // something wrong
    }
}