Perhaps it's because you're using the custom options object - you need to use the configurable options object to pass through configurable attributes using the "configurable_item_options" object.

{
  "cartItem": {
    "item_id": 0,
    "sku": "string",
    "qty": 0,
    "name": "string",
    "price": 0,
    "product_type": "string",
    "quote_id": "string",
    "product_option": {
    "extension_attributes": {
       "configurable_item_options": [
         {
          "option_id": "string",
          "option_value": 0,
          "extension_attributes": {}
        }
      ]
    }
  },
  "extension_attributes": {}
  }
}

Perhaps it's because you're using the custom options object - you need to use the "configurable_item_options" object to pass through the configurable attributes.

{
  "cartItem": {
    "item_id": 0,
    "sku": "string",
    "qty": 0,
    "name": "string",
    "price": 0,
    "product_type": "string",
    "quote_id": "string",
    "product_option": {
    "extension_attributes": {
       "configurable_item_options": [
         {
          "option_id": "string",
          "option_value": 0,
          "extension_attributes": {}
        }
      ]
    }
  },
  "extension_attributes": {}
  }
}

It's because you're using the custom options object - you need to use the configurable options object to pass through configurable attributes using the "configurable_item_options" object.

{
  "cartItem": {
    "item_id": 0,
    "sku": "string",
    "qty": 0,
    "name": "string",
    "price": 0,
    "product_type": "string",
    "quote_id": "string",
    "product_option": {
    "extension_attributes": {
       configurable_item_options": [
       {
          "option_id": "string",
          "option_value": 0,
          "extension_attributes": {}
       }
     ]
   }
},
"extension_attributes": {}

} }

Perhaps it's because you're using the custom options object - you need to use the configurable options object to pass through configurable attributes using the "configurable_item_options" object.

{
  "cartItem": {
    "item_id": 0,
    "sku": "string",
    "qty": 0,
    "name": "string",
    "price": 0,
    "product_type": "string",
    "quote_id": "string",
    "product_option": {
    "extension_attributes": {
       "configurable_item_options": [
         {
          "option_id": "string",
          "option_value": 0,
          "extension_attributes": {}
        }
      ]
    }
  },
  "extension_attributes": {}
  }
}