2签出退款问题

使用 2Checkout's supported library for PHP ,我无法使用他们提供的示例和我自己的凭据通过他们的API在他们的沙箱中进行退款。我做错了什么?请注意,我可以在他们的沙箱中进行收费,但不能退款。

<?php

error_reporting(E_ALL);
ini_set('display_errors','On');
header('Content-Type: text/plain');

require_once('lib/Twocheckout.php');

$sChargeID = '4093729353592'; // my order ID
Twocheckout::username('USER'); // my username
Twocheckout::password('PASSWORD'); // my password
Twocheckout::sandbox(true);

Twocheckout::verifySSL(false);

$charge = Twocheckout_Sale::refund(array(
    'sale_id' => $sChargeID,
    'comment' => 'Customer decided the item did not meet expectations and it was within their refund period.',
    'category' => 2 // did not like item
));

echo "\nRESPONSE = \n";
print_r($charge);
echo "\n";

当我转储CURL响应时,我看到以下内容:

{
   "errors" : [
      {
         "code" : "FORBIDDEN",
         "message" : "Access denied to API"
      }
   ]
}