Curl request works on localhost but not when I upload to Ubuntu VPS

$date = date( 'Y/m/d H:i' );
$xmldata = '
     <API3G>
     <CompanyToken>545415541</CompanyToken>
     <Request>createToken</Request>
     <Transaction>
     <PaymentAmount>400</PaymentAmount>
     <PaymentCurrency>NAD</PaymentCurrency>
     <CompanyRef>888ff </CompanyRef>

     <RedirectURL>https://secure.3gdirectpay.com/payv2.php?ID=token</RedirectURL>
     <BackURL>www.motlee-systems.com</BackURL>
     <customerEmail>jeoemex@gmail.com</customerEmail>
     </Transaction>
     <Services>
     <Service>
     <ServiceType>3854</ServiceType>
     <ServiceDescription>me</ServiceDescription>
     <ServiceDate>2020/11/06</ServiceDate>
     </Service>
     </Services>
     </API3G>';
        
$url = "https://example.com/API/v6/";

$ch = curl_init();
if (!$ch) {
    die("Couldn't initialize a cURL handle");
}

curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_TIMEOUT, 60);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: text/xml'));
curl_setopt($ch, CURLOPT_POSTFIELDS, $xmldata);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
$result = curl_exec($ch); // execute
echo $result;             //show response
curl_close($ch);

if ( $result != '' ) {
    $xml = new \SimpleXMLElement( $result );

    // Check if token was created successfully
    if ( $xml->xpath( 'Result' )[0] != '000' ) {
        exit("me");
    } else {                
        $transToken = $xml->xpath( 'TransToken' )[0]->__toString();
        $result = $xml->xpath( 'Result' )[0]->__toString();
        $resultExplanation = $xml->xpath( 'ResultExplanation' )[0]->__toString();
        $transRef = $xml->xpath( 'TransRef' )[0]->__toString();
    }

 } else {
     throw new \Exception( 'Token could not be created. Please go back and try again' );
 }

 header("Location: https://example.com/payv2.php?ID=$transToken");

The code above runs perfectly on localhost / XAMPP but when I upload this file on my Ubuntu 16.04 VPS, it's giving this error:

This page isn’t working www.example.com is currently unable to handle this request. HTTP ERROR 500

Any idea why? I've installed curl and php-xml on Ubuntu but nothing.