Magento – How to add quote to customer in magento2

I want to set custom pricing for magento2 product through API . I googled for the same and tried to implement the solutions one by one. But no solution is worked out.

$cartId = $this->cartManagementInterface->createEmptyCartForCustomer($customer_id); //Create empty cart
$quote = $this->cartRepositoryInterface->get($cartId);

I am getting error on creating empty cart for customer.

"Warning: Illegal offset type in isset or empty in /var/www/html/mypro/vendor/magento/module-quote/Model/QuoteRepository.php on line 133"

Can any one help me on this? I am adding complete code that i am trying. Please help me on this.

class PoApi implements PosApiInterface
{
    public function __construct(
        \Magento\Framework\App\RequestInterface $request,
        \Magento\Framework\ObjectManagerInterface $objectManager,
        \My\RequestToQuote\Model\Quote $quote,
        \My\RequestToQuote\Model\QuoteDetail $qdetails,
        \My\RequestToQuote\Model\Po $po,
        \My\Vincrequest\Model\Vpo $vpo,
        \My\RequestToQuote\Model\PoDetail $podetails,
        \Magento\Quote\Api\CartRepositoryInterface $cartRepositoryInterface,
        \Magento\Quote\Api\CartManagementInterface $cartManagementInterface,
        \Magento\Framework\Registry $registry,
        \Magento\Framework\UrlInterface $urlinterface,
        \Magento\Customer\Api\CustomerRepositoryInterfaceFactory $customerRepositoryFactory
    ) {
        $this->_request = $request;
        $this->_objectManager = $objectManager;
        $this->_quote = $quote;
        $this->_quotedetails = $qdetails;
        $this->_po = $po;
        $this->_vpo = $vpo;
        $this->_podetails = $podetails;
        $this->_coreRegistry = $registry;
        $this->_urlBuilder = $urlinterface;
        $this->_customerRepository = $customerRepositoryFactory;
        $this->cartRepositoryInterface=$cartRepositoryInterface;
        $this->cartManagementInterface=$cartManagementInterface;
    }

    public function poToCart($poid)
    {
        try {
            if (!isset($poid) || $poid == '' || empty($poid)) {
                return "Po Id Is Required";
            }

            $po_incId = $poid;
            $poData= $this->_po->load($po_incId, 'po_increment_id');
            $status = $poData->getStatus();
            $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
            $customerObj = $objectManager->create('Magento\Customer\Model\Customer')->load($poData->getPoCustomerId());
            $quote_id = $poData->getQquoteId();
            //  $quote    = $this->quoteModel->loadByCustomer($customer_id);
            if ($status == '1' || $status == '0') {
                $poData->setData('status', '1');
                $poData->save();

                $podetail = $poData->getCollection()->addFieldToFilter('po_increment_id', $po_incId)->addFieldToFilter('status', '1')->getData();

                if (sizeof($podetail)>0) {
                    $poProd=$this->_objectManager->create('My\Crequest\Model\PoDetail')->getCollection()->addFieldToFilter('po_id', $po_incId)->getData();


                    $cartId = $this->cartManagementInterface->createEmptyCart(); //Create empty cart
                    $quote = $this->cartRepositoryInterface->get($cartId); // load empty cart quote
                    $quote->setStore($store);
                    // if you have allready buyer id then you can load customer directly
                    $customer= $this->customerRepository->getById($customer->getEntityId());
                    $quote->setCurrency();
                    $quote->assignCustomer($customer); //Assign quote to customer

                    //add items in quote
                    foreach ($poProd as $item) {
                        $product=$this->_product->load($item['product_id']);
                        $product->setPrice($item['po_price']);
                        $quote->addProduct($product, intval($item['product_qty']));
                    }

                    $quote->save(); //Now Save quote and your quote is ready

                    return "Added Cart";
                }
            }
        } catch (\Exception $ex) {
            return $ex->getMessage();
        }
    }
}

When i tried to call this API. I am getting below error message.

Magento\Framework\Exception\CouldNotSaveException: Cannot create quote in /var/www/html/myproj/vendor/magento/module-quote/Model/QuoteManagement.php:239
Stack trace:
#0 /var/www/html/myproj/app/code/My/Crequest/Model/PoApi.php(316): Magento\Quote\Model\QuoteManagement->createEmptyCart()
#1 [internal function]: My\Crequest\Model\PoApi->poToCart('PO000101')

I tried another code as well by hitting API programmatically.

$store=$this->_storeManager->getStore();
$repository = $this->_objectManager->create(\Magento\Customer\Api\CustomerRepositoryInterface::class);
/** @var $customer \Magento\Customer\Api\Data\CustomerInterface */
$customer = $repository->getById(2);
$customerId = $customerObj->getId();

$serviceInfo = [
    'rest' => [
        'resourcePath' => '/V1/customers/' . $customerId . '/carts',
        'httpMethod' => \Magento\Framework\Webapi\Rest\Request::HTTP_METHOD_POST,
    ]
];

echo $quoteId = $this->_webApiCall($serviceInfo, ['customerId' => $customerId]);

But this is also giving an error like below.

 Uncaught Error: Call to undefined method My\Crequest\Model\PoApi::_webApiCall() in