Omnipay Paypal Express Chekout Error: Security header is not valid

I have searched around stack overflow and google in general and have not found a problem similar to mine.

The problem is whenever setTestMode() method is used the error "Security header is not valid" pops up, but if I remove it and just keep the setUsername(), setPassword(), and setSignature() methods, it goes through and redirects straight to paypal (Live Paypal).

I am currently using Laravel 5.8 with Omnipay/paypal using Paypal Express Checkout

Here are the files that were used

Paypal.php

public function gateway()
{
    $gateway = Omnipay::create('PayPal_Express');

    $gateway->setUsername(config('services.paypal.username'));
    $gateway->setPassword(config('services.paypal.password'));
    $gateway->setSignature(config('services.paypal.signature'));
    $gateway->setTestMode(config('services.paypal.sandbox'));
    // $gateway->setTestMode(true);

    return $gateway;
}

public function purchase(array $parameters)
{
    $response = $this->gateway()
        ->purchase($parameters)
        ->send();

    return $response;
}

PaypalController.php

public function checkout($order_id)
{
    $order = Order::findOrFail(decrypt($order_id));

    $paypal = new PayPal;

    $response = $paypal->purchase([
        'amount' => $paypal->formatAmount($order->amount),
        'transactionId' => $order->transaction_id,
        'currency' => 'PHP',
        'cancelUrl' => $paypal->getCancelUrl($order),
        'returnUrl' => $paypal->getReturnUrl($order),
        'notifyUrl' => $paypal->getNotifyUrl($order),
    ]);

    if ($response->isRedirect()) {
        $response->redirect();
    }

    return redirect()->back()->with([
            'message' => $response->getMessage(),                
    ]);
}

Here are the contents of the $response

ExpressAuthorizeResponse {#1098 ▼
  #liveCheckoutEndpoint: "https://www.paypal.com/cgi-bin/webscr"
  #testCheckoutEndpoint: "https://www.sandbox.paypal.com/cgi-bin/webscr"
  #request: ExpressAuthorizeRequest {#1095 ▼
    #liveEndpoint: "https://api-3t.paypal.com/nvp"
    #testEndpoint: "https://api-3t.sandbox.paypal.com/nvp"
    #negativeAmountAllowed: true
    #parameters: ParameterBag {#1097 ▶}
    #httpClient: Client {#1063 ▶}
    #httpRequest: Request {#1086 ▶}
    #response: ExpressAuthorizeResponse {#1098}
    #currencies: ISOCurrencies {#1096}
    #zeroAmountAllowed: true
  }
  #data: array:9 [▼
    "TIMESTAMP" => "2020-02-03T11:04:45Z"
    "CORRELATIONID" => "c8d066c9b5ccd"
    "ACK" => "Failure"
    "VERSION" => "119.0"
    "BUILD" => "54118205"
    "L_ERRORCODE0" => "10002"
    "L_SHORTMESSAGE0" => "Security error"
    "L_LONGMESSAGE0" => "Security header is not valid"
    "L_SEVERITYCODE0" => "Error"
  ]
}

via Chebli Mohamed