CodeIgniter，Gocart中的Paypal Express Checkout错误（不支持指定的方法）

我是codeigniter和paypal的新手 . 我正在研究gocart（一种基于codeIgniter构建的开源电子商务解决方案） . 我尝试使用集成在其中的paypal API，但其显示错误如下： [ACK] => Failure [L_ERRORCODE0] => 81002 [L_SHORTMESSAGE0] => Unspecified Method [L_LONGMESSAGE0] => Method Specified is not Supported [L_SEVERITYCODE0] => Error
以下是我的代码： paypal_expres.php

function __construct() 
{ 
$this->API_USERNAME ='Username'; 
$this->API_PASSWORD = 'Password'; 
$this->API_SIGNATURE ='Signature'; 
$this->RETURN_URL = 'www.example.com'; 
$this->CANCEL_URL = 'www.example.com'; 
$this->currency = 'USD';
$this->host = "api-3t.sandbox.paypal.com"; 
$this->gate = 'https://www.sandbox.paypal.com/cgi-bin/webscr?';
} 

 public function doExpressCheckout($amount, $desc, $invoice='')
{ 
    $data = array(
    'PAYMENTACTION' =>'Sale',    'AMT' => '24',    'RETURNURL' => $this->getReturnTo(),
    'CANCELURL'  => $this->getReturnToCancel(),    'CURRENCYCODE'=> $this->currency,    'METHOD' => 'SetExpressCheckout');    

 $query = $this->buildQuery($data); 
$result = $this->response($query); 
$response = $result->getContent(); 
$return = $this->responseParse($response); 
echo ''; 
print_r($return); 
echo ''; 
if ($return['ACK'] == 'Success') 
{ 
header('Location: '.$this->gate.'cmd=_express-checkout&useraction=commit&token='.$return['TOKEN'].''); 
die();    }   

 return($return);} 

 public function doExpressCheckout($amount, $desc, $invoice='')
{ 
$data = array(    'PAYMENTACTION' =>'Sale',    'AMT' => '24',    'RETURNURL' => $this->getReturnTo(),    'CANCELURL'  => $this->getReturnToCancel(),    'CURRENCYCODE'=> $this->currency,    'METHOD' => 'SetExpressCheckout');    

 $query = $this->buildQuery($data); 
$result = $this->response($query); 
$response = $result->getContent(); 
$return = $this->responseParse($response); 
echo ''; 
print_r($return); 
echo '';    

 if ($return['ACK'] == 'Success') 
{ 
header('Location: '.$this->gate.'cmd=_express-checkout&useraction=commit&token='.$return['TOKEN'].''); 
die();    }    

 return($return);
} 

 private function response($data)
{ 
$result = $this->CI->httprequest->connect($data); 
if ($result<400) 
return 
$this->CI->httprequest; 
return false;} 

 private function buildQuery($data = array())
{ 
$data['USER'] = $this->API_USERNAME; 
$data['PWD'] = $this->API_PASSWORD; 
$data['SIGNATURE'] = $this->API_SIGNATURE; 
$data['VERSION'] = '56.0'; 
$query = http_build_query($data); 
return $query;
}