I am trying to add multiple configurable products to cart using a custom REST API request. For that I call the existing save function for each products. I’m able to add upto product options. But when I try to add extension attributes, it’s is not working as expected. Instead of that, I’m getting error message "Fatal Error: 'Uncaught Error: Cannot instantiate interface Magento\Quote\Api\Data\CartItemExtensionInterface in ../vendor/magento/framework/ObjectManager/Factory/AbstractFactory.php:9

Whenever I try to include \Magento\Quote\Api\Data\CartItemExtensionInterface in constructor, this error message is alone occurs. I am doing like below

use Magento\Quote\Api\Data\CartItemExtensionInterface; use Magento\Framework\Api\AttributeValueFactory; use Magento\Framework\Api\ExtensionAttributesFactory; use Magento\Quote\Api\Data\CartItemInterface;  class MultiItemsCart extends \Magento\Quote\Model\Quote\Item\Repository implements MultiItemsCartInterface {     /**      * Quote repository.      *      * @var \Magento\Quote\Api\CartRepositoryInterface      */     protected $  quoteRepository;     protected $  serviceProcessor;     protected $  quoteItemRepository;     /**      * @var \Magento\Quote\Api\Data\CartItemInterfaceFactory      */     public $  cartItem;     public $  productOption;     public $  extensionAttributes;      /**      * @param \Magento\Quote\Api\CartRepositoryInterface $  quoteRepository      */     public function __construct(         \Magento\Quote\Api\CartRepositoryInterface $  quoteRepository,         \Magento\Framework\Webapi\ServiceInputProcessor $  serviceProcessor,         \Magento\Quote\Model\Quote\Item\Repository $  quoteItemRepository,         \Magento\Quote\Api\Data\CartItemInterface $  cartItem,         \Magento\Quote\Api\Data\ProductOptionInterface $  productOption,         \Magento\Quote\Api\Data\CartItemExtensionInterface $  extensionAttributes,         array $  cartItemProcessors = []     ) {         $  this->quoteRepository = $  quoteRepository;         $  this->serviceProcessor = $  serviceProcessor;         $  this->quoteItemRepository =$  quoteItemRepository;         $  this->itemDataFactory = $  itemDataFactory;         $  this->cartItem = $  cartItem;         $  this->productOption = $  productOption;         $  this->extensionAttributes = $  extensionAttributes;     }     /**     * Returns greeting message to user     *     * @api     * @param mixed $  cartItems     * @return array     */     public function multiItemsCart($  cartItems) {         foreach($  cartItems as $  cartItem){             $  this->cartItem->setSku($  cartItem['sku']);             $  this->cartItem->setQuoteId($  cartItem['quote_id']);             $  this->cartItem->setQty($  cartItem['qty']);             $  this->cartItem->setProductOption($  this->productOption);             //$  this->cartItem->setExtensionAttributes($  this->extensionAttributes);             $  this->quoteItemRepository->save($  this->cartItem);          }         return $  cartItems;     } } 

I tried to include the same class in some other class also, there also I’m getting same error message. Can any one help on this…