{"code":4,"message":"Invalid value passed for bulk_update"} while creating invoice

{"code":4,"message":"Invalid value passed for bulk_update"} while creating invoice

I am trying to create invoice on submitting a form built in PHP and below is my code :

curl_setopt($ch, CURLOPT_URL, "https://books.zoho.in/api/v3/invoices?organization_id=60005986825");
        // curl_setopt($ch, CURLOPT_POST, 1);
        // curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($post_data));
        // curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        // curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
        curl_setopt($ch, CURLOPT_HTTPHEADER, array(
            'Authorization: Zoho-oauthtoken '.$access_token,
            'Content-Type: application/x-www-form-urlencoded'
        ));
        // $data = array(
            // 'authtoken'   => $access_token,
            // 'JSONString'   => json_encode($post_data),

            // "organization_id"  => '60005986825',
            // "send"                  => 'false'
        // );

        curl_setopt($ch, CURLOPT_VERBOSE, 1);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
        curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($post_data));
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
        curl_setopt($ch, CURLOPT_POST, TRUE);
        // curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json") );
        $responseinsert = curl_exec($ch);
        // $responseinsert = json_decode($response);
        echo '<pre>'; print_r($responseinsert); echo '</pre>';
    }
}
insert_leads();
?>
<form action="" method="post" name="form">   
    <table>
        <tr>
            <td>Company</td>
            <td><input type="text" name="company" required/></td>
        </tr>
        <tr>
            <td>Last Name</td>
            <td><input type="text" name="lname" required/></td>
        </tr>
        <tr>
            <td>First Name</td>
            <td><input type="text" name="fname" required/></td>
        </tr>
        <tr>
            <td>Email</td>
            <td><input type="email" name="email" required/></td>
        </tr>
        <tr>
            <td>State</td>
            <td><input type="text" name="state" required/></td>
        </tr>
        <tr>
            <td>Invoice Amount</td>
            <td><input type="number" name="amt" required/></td>
        </tr>
        <tr>
            <td>Address</td>
            <td><input type="text" name="addr" required/></td>
        </tr>
        <tr>
            <td><input type="submit" name="submit" value="Submit"/></td>
        </tr>
    </table> 
</form>