Magento 2 – How to Get Cart ID

You can use ManagerInterface for display message in magento 2

..........
protected $_messageManager;

public function __construct(
    \Magento\Framework\Message\ManagerInterface $messageManager
) {
    $this->_messageManager = $messageManager;
}

public function yourmethod() {
    ..
    $message = 'You need to add more {$quantityGroup->getTitle()}';
    $this->_messageManager->addSuccess($message); //For Success Message
    $this->_messageManager->addError($message);//For Error Message
    ..
}

This way only for registered customers:

Request path - http://127.0.0.1:8000/rest/V1/carts/3/items

Where 3 is your cart id

Method: POST

BODY: { "cartItem" : { "sku": "24-MB01", "qty": "1", "quote_id": 3 } }

Cart Id ( quote_id ) you can retrieve via:

http://127.0.0.1:8000/rest/V1/customers/1/carts

Method: POST

The response will be similar to:

"3"

Checked via Postman