php - Paypal Express Checkout Error in CodeIgniter, Gocart(Method Specified is not Supported) -

i new codeigniter , paypal. working on gocart(an open source ecommerce solution built on codeigniter). try work on paypal api integrated in it, showing error follows : [ack] => failure [l_errorcode0] => 81002 [l_shortmessage0] => unspecified method [l_longmessage0] => method specified not supported [l_severitycode0] => error
below code : 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; }