Laravel Paypal via Omnipay - Payee account is invalid - laravel

Question

I am trying to submit a payment against Paypal Sandbox, using the Omnipay Paypal module.
I think that I have everything set up in terms of sandbox account, and my transaction is working if I don't specify a card as payment method. I am correctly redirected to the Paypal sandbox, where I need to enter my test credentials before I can submit my payment. Then I am redirected back to my original web site, where I can store the confirmed payment details.
However, when I want to use a credit card, I get an error message saying : Payee account is invalid.
Here is my controller code :
<?php
namespace App\Http\Controllers;
use Illuminate\Http\Request;
use Omnipay\Omnipay;
use Omnipay\Common\CreditCard;
use App\Payment;
class PaymentController extends Controller
{
private $gateway;
public function __construct()
{
$this->gateway = Omnipay::create('PayPal_Rest');
$this->gateway->setClientId(env('PAYPAL_CLIENT_ID'));
$this->gateway->setSecret(env('PAYPAL_CLIENT_SECRET'));
$this->gateway->setTestMode(env('PAYPAL_SANDBOX_MODE'));;
}
/**
* Call a view.
*/
public function index()
{
return view('payment');
}
/**
* Initiate a payment on PayPal.
*
* #param \Illuminate\Http\Request $request
*/
public function charge(Request $request)
{
if($request->input('submit'))
{
$card = new CreditCard(array(
'number' => '<my test credit card number>',
'expiryMonth' => '03',
'expiryYear' => '2024',
'cvv' => '<my ccv>',
));
try {
$response = $this->gateway->purchase(array(
'amount' => $request->input('amount'),
'currency' => env('PAYPAL_CURRENCY'),
'returnUrl' => url('success'),
'cancelUrl' => url('error'),
'card' => $card
))->send();
if ($response->isRedirect()) {
$response->redirect(); // this will automatically forward the customer
} else {
// not successful
return $response->getMessage();
}
} catch(Exception $e) {
return $e->getMessage();
}
}
}
/**
* Charge a payment and store the transaction.
*
* #param \Illuminate\Http\Request $request
*/
public function success(Request $request)
{
// Once the transaction has been approved, we need to complete it.
if ($request->input('paymentId') && $request->input('PayerID'))
{
$transaction = $this->gateway->completePurchase(array(
'payer_id' => $request->input('PayerID'),
'transactionReference' => $request->input('paymentId'),
));
$response = $transaction->send();
if ($response->isSuccessful())
{
// The customer has successfully paid.
$arr_body = $response->getData();
// Insert transaction data into the database
$payment = new Payment;
$payment->payment_id = $arr_body['id'];
$payment->payer_id = $arr_body['payer']['payer_info']['payer_id'];
$payment->payer_email = $arr_body['payer']['payer_info']['email'];
$payment->amount = $arr_body['transactions'][0]['amount']['total'];
$payment->currency = env('PAYPAL_CURRENCY');
$payment->payment_status = $arr_body['state'];
$payment->save();
return "Payment is successful. Your transaction id is: ". $arr_body['id'];
} else {
return $response->getMessage();
}
} else {
return 'Transaction is declined';
}
}
/**
* Error Handling.
*/
public function error()
{
return 'User cancelled the payment.';
}
}
Can you see where my problem could be ?