Error 500 (Internal Server Error) with PHP AJAX | Contact Form

I have created a contact form and every time I give it to send with all the fields filled , the following error appears:

HTML

<form id="contact-form" method="POST" action="">
<input type="text" id="form-name" name="form-name" placeholder="Nombre" required>
<input type="email" id="form-email" name="form-email" placeholder="Email" required>
<textarea id="form-message" name="form-message" placeholder="Mensaje" required></textarea>
<button type="submit" id="contact-btn" class="contact-btn btn">Enviar</button>
</form>

JQUERY | AJAX

$(function() {
    var form = $('#contact-form');
    var formMessages = $('#error-text');

    $(form).submit(function(event) {
        event.preventDefault();
        var formData = $(form).serialize();

        $.ajax({
            type: 'POST',
            url: 'include/php/contact.php',
            data: formData
        }).done(function(response) {
            $(formMessages).text(response);
            $('#error-alert').removeClass('closed');
            setTimeout(function(){ $('#error-alert').addClass('closed'); },5000);

            $('#form-name').val('');
            $('#form-email').val('');
            $('#form-message').val('');
        }).fail(function(data) {
            $(formMessages).text(response);
            $('#error-alert').removeClass('closed');
            setTimeout(function(){ $('#error-alert').addClass('closed'); },5000);

            if (data.responseText !== '') {
                $(formMessages).text(data.responseText);
            } else {
                $(formMessages).text('<p>Oops! Ocurrió un error y tu mensaje no pudo ser enviado...</p>');
            }
        });
    });
});

PHP

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $name = strip_tags(trim($_POST["form-name"]));
    $name = str_replace(array("\r","\n"),array(" "," "),$name);
    $email = filter_var(trim($_POST["form-email"]), FILTER_SANITIZE_EMAIL);
    $message = trim($_POST["form-message"]);

    if (empty($name) OR empty($message) OR !filter_var($email, FILTER_VALIDATE_EMAIL)) {
        http_response_code(400);
        echo "<p>Oops! Hubo un problema con tu envío. Por favor, completa el formulario e inténtalo otra vez...</p>";
        exit;
    }

    $recipient = "[email protected]";
    $subject = "Nuevo mensaje de $name";

    $email_content = "Nombre: $name\n";
    $email_content .= "Email: $email\n\n";
    $email_content .= "Mensaje:\n$message\n";

    $email_headers = "De: $name <$email>";

    if (mail($recipient, $subject, $email_content, $email_headers)) {
        http_response_code(200);
        echo "<p>¡Gracias! Tu mensaje ha sido enviado</p>";
    } else {
        http_response_code(500);
        echo "<p>Oops! Hubo un problema y no pudimos enviar tu mensaje. Por favor, inténtalo más tarde...</p>";
    }
} else {
    http_response_code(403);
    echo "<p>Hubo un problema con tu envío. Por favor, inténtalo otra vez...</p>";
}
?>

It says that the variable response is not defined ...

Thank you very much for your help