paypal permission error when using omnipay with laravel

Question

I have used this in my project, hoping to integrate paypal express.

but I've got this error:

You do not have permissions to make this API call

here is my controller:

public function index()
{
    $gateway = Omnipay::create('PayPal_Express');
    $gateway->setUsername('xxxxxx');
    $gateway->setPassword('xxxxxx');

    $formData = [
        'number' => '4242424242424242',
        'expiryMonth' => '6',
        'expiryYear' => '2016',
        'cvv' => '123'
    ];

    $response = $gateway->purchase(
        [
            'amount' => '1.00',
            'currency' => 'USD',
            'card' => $formData,
            'returnUrl'     => '/checkout/',
            'cancelUrl'     => '/cancel/',
        ]
    )->send();

    // Process response
    if ($response->isSuccessful()) {

        // Payment was successful
        print_r($response);

    } elseif ($response->isRedirect()) {

        // Redirect to offsite payment gateway
        $response->redirect();

    } else {

        // Payment failed
        echo $response->getMessage();
    }
}

I have enabled api access in my paypal account but got same result. I assume the gateway login details for setUsername and setPassword are the details provided by paypal after setting up the api. I have also changed these details with my paypal login but I've got the same result.