「致命的なエラー: クラス 'PHPMailer' が見つかりません」エラーが表示されるのはなぜですか? どのように修正すればよいですか?

require("path/to/PHPMailer.php");
require("path/to/SMTP.php");

$mail = new PHPMailer\PHPMailer\PHPMailer();

$mail->IsSMTP();
$mail->SMTPDebug = 1;
$mail->SMTPAuth = true;
$mail->SMTPSecure = 'ssl';
$mail->Host = "smtp.gmail.com";
$mail->Port = 465;
$mail->IsHTML(true);
$mail->Username = "your-email";
$mail->Password = "your-password";
$mail->SetFrom("from@email.com");
$mail->Subject = "Subject";
$mail->Body = "Message";
$mail->AddAddress("to@email.com");

if(!$mail->Send()) {
    echo "Mailer Error: " . $mail->ErrorInfo;
} else {
    echo "Message has been sent";
}