PHPmailer ส่ง email ไม่ได้ และ ไม่ทราบ error ทำไงดีครับ

<?php
require './vendor/autoload.php';
$mail = new PHPMailer();

$mail->SMTPDebug = 0;                               // Enable verbose debug output
//echo 'create email...';
$mail->IsSMTP();                                      // Set mailer to use SMTP
$mail->Host = 'smtp.gmail.com';  // Specify main and backup SMTP servers
$mail->SMTPAuth = true;                               // Enable SMTP authentication
$mail->Username = '[email protected]';                 // SMTP username
$mail->Password = 'xxxxxxx';                           // SMTP password
$mail->SMTPSecure = 'tls';                            // Enable TLS encryption, `ssl` also accepted
$mail->Port = 587;                                    // TCP port to connect to
//echo 'setting....';
$mail->setFrom('[email protected]', 'Mailer');
$mail->addAddress('[email protected]', 'Joe User');     // Add a recipient
$mail->addAddress('[email protected]');               // Name is optional


//$mail->addAttachment('/var/tmp/file.tar.gz');         // Add attachments
//$mail->addAttachment('/tmp/image.jpg', 'new.jpg');    // Optional name
$mail->isHTML(true);                                  // Set email format to HTML

$mail->Subject = 'Here is the subject';
$mail->Body    = 'This is the HTML message body <b>in bold!</b>';
$mail->AltBody = 'This is the body in plain text for non-HTML mail clients';

//echo 'sending...';


$result = $mail -> send();
//echo 'result :'.$result;

if(!$mail->send()) {
    echo 'Message could not be sent.';
    echo 'Mailer Error: ' . $mail->ErrorInfo;
} else {
    echo 'Message has been sent';
}
?>