PHPmailer Hata

use PHPMailer\PHPMailer\PHPMailer;
  use PHPMailer\PHPMailer\Exception;

  require 'PHPMailer/src/Exception.php';
  require 'PHPMailer/src/PHPMailer.php';
  require 'PHPMailer/src/SMTP.php';

    $mail = new PHPMailer(true);


try{
    $mail->isSMTP();
    $mail->SMTPKeepAlive = true;
    $mail->SMTPAuth = true;
    $mail->SMTPSecure = 'tls'; //ssl

    $mail->Port = 587; //25 , 465 , 587
    $mail->Host = "smtp.gmail.com";

    $mail->UserName = "demo@gmail.com";
    $mail->Password = "sifre";


    $mail->setFrom("alici@gmail.com");
    $mail->addAddress("alici@gmail.com");


    $mail->isHTML(true);
    $mail->Subject = "Gmail SMTP Ornegi";
    $mail->Body = "<h1>Merhaba </h1><p>Bu bir denemedir.</p>";

    #$mail->addAttachment("dosya.txt");

    $mail->send();
   echo 'Message has been sent';
} catch (Exception $e) {
   echo "Message could not be sent. Mailer Error: {$mail->ErrorInfo}";
}

$mail->Password = "sif\.re";