Get json response through custom api endpoint

I am creating a custom api endpoint to receive some data in json format. This is my module’s code

webapi.xml

<?xml version="1.0" ?>
<routes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Webapi:etc/webapi.xsd">
<route url="/V1/matrid-nextapi/orders" method="POST">
    <service class="MatridNextApiApiOrdersManagementInterface" method="postOrders"/>
    <resources>
        <resource ref="anonymous"/>
    </resources>
</route>

di.xml

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="MatridNextApiApiOrdersManagementInterface" type="MatridNextApiModelOrdersManagement"/>
</config>

Api/OrdersManagementInterface.php

<?php
/**
 * Copyright ©  All rights reserved.
 * See COPYING.txt for license details.
 */
namespace MatridNextApiApi;

interface OrdersManagementInterface
{

    /**
     * 
     * @param mixed $data
     * @return mixed[]
     */
    public function postOrders($data);
}

Model/OrdersManagement.php

<?php
/**
 * Copyright ©  All rights reserved.
 * See COPYING.txt for license details.
 */

namespace MatridNextApiModel;

class OrdersManagement implements MatridNextApiApiOrdersManagementInterface
{

    /**
    *
    * @param mixed $data
    * @return mixed[]
    */
    public function postOrders($data)
    {
        $data = json_decode($data);
        echo "<pre>"; print_r($data);
    }
}

Now I have created a script on magento root and try to hit this api using curl. I am expecting to get the posted array as result but I am getting an error message as response.

This is my custom script to check that is placed on magento root.

**custom-script.php**

<?php
$data = array (
      "Orders"=> [
            "ID"=> 1000006636,
            "Brand"=> "Brand",
      ]
    );

$ch = curl_init("https://staging.nonnon.co.uk/index.php/rest/V1/matrid-nextapi/orders");
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($data));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json", "Content-Lenght: " . strlen(json_encode($data))));

$result = curl_exec($ch);

echo "<pre>";print_r($result);

?>

This script is giving me output as

{"message":""%fieldName" is required. Enter and try again.","parameters":{"fieldName":"data"}}

I need the $data array as the output. I am not sure what I have missed as this is my first time working with magento apis. Could someone guide me in the right direction.
I just need $data as the response and then I will manipulate it according to my requirements.