Adding Groped Products items to the cart using the Api

I am using this endpoint :

http://mage.local/rest/V1/carts/3747/items

and this is the body:

{
  "cartItem": {
    "sku": "8717752030514",
    "qty": 1,
    "quote_id": "3747"
  }
}

I have checked the above associated product have the in stock, also have quantity.
But when I do this I am getting this :

Product you are trying to add is not available

My parent sku is demo_8717752030514.

All things are available in the Magento instance.

I want to place the order for the grouped products.