PHP Form not showing Success or Error messages when submitted

I checked so many posts on here but I am unable to fix this. I have a PHP form and although the email is sent it doesn't show any massage and I don't know what I am missing.

In the PHP it mentions a success and failed but no reference on where that message sits? I set up PHP forms before but this is throwing me off.

Anyone can help, please?

The sendmail.php code

<?php

// Define some constants
define( "RECIPIENT_NAME", "Email" );
define( "RECIPIENT_EMAIL", "email@email.com" );


// Read the form values
$success = false;
$senderName = isset( $_POST['username'] ) ? preg_replace( "/[^\s\S\.\-\_\@a-zA-Z0-9]/", "", $_POST['username']) : "";
$senderEmail = isset( $_POST['email'] ) ? preg_replace( "/[^\.\-\_\@a-zA-Z0-9]/", "", $_POST['email'] ) : "";
$message = isset( $_POST['message'] ) ? preg_replace( "/(From:|To:|BCC:|CC:|Subject:|Content-Type:)/", "", $_POST['message'] ) : "";

// If all values exist, send the email
if ( $senderName && $senderEmail && $message) {
  $recipient = RECIPIENT_NAME . " <" . RECIPIENT_EMAIL . ">";
  $headers = "From: " . $senderName . "";
  $msgBody = " Email: ". $senderEmail . " Message: " . $message . "";
  $success = mail( $recipient, $headers, $msgBody );

  //Set Location After Successsfull Submission
  header('Location: contact.html?message=Successfull');
}

else{
    //Set Location After Unsuccesssfull Submission
    header('Location: contact.html?message=Failed');    
}

?>

The HTML code of the contact.html:

 <!--Contact Section-->
    <section class="contact-section">
        <div class="auto-container">

            <div class="info-container">
                <div class="row clearfix">
                </div>
            </div>

            <div class="contact-container">
                <div class="row clearfix">
                    <!--Form-->
                    <div class="form-column col-lg-6 col-md-12 col-sm-12">
                        <div class="inner wow fadeInLeft" data-wow-delay="0ms" data-wow-duration="1500ms">
                            <div class="sec-title">
                                <h2>Send <strong>Your Message</strong></h2>
                                <div class="lower-text">Don’t Hesitate to Contact Us</div>
                            </div>
                            <div class="default-form contact-form">
                                <form method="post" id="contact-form" action="sendemail.php">
                                    <div class="row clearfix">          
                                        <div class="form-group col-lg-6 col-md-6 col-sm-12">
                                            <div class="field-label">Name</div>
                                            <div class="field-inner">
                                                <input type="text" name="username" placeholder="Your Name" required="" value="">
                                            </div>
                                        </div>

                                        <div class="form-group col-lg-6 col-md-6 col-sm-12">
                                            <div class="field-label">Email</div>
                                            <div class="field-inner">
                                                <input type="email" name="email" placeholder="Email Address" required="" value="">
                                            </div>
                                        </div>

                                        <div class="form-group col-lg-12 col-md-12 col-sm-12">
                                            <div class="field-label">Message</div>
                                            <div class="field-inner">
                                                <textarea name="message" placeholder="Write your message..." required=""></textarea>
                                            </div>
                                        </div>
                
                                        <div class="form-group col-lg-12 col-md-12 col-sm-12">
                                            <button type="submit" class="theme-btn btn-style-three"><span class="btn-title">Send Your Message</span></button>
                                        </div>
                                    </div>
                                </form>
                            </div>
                        </div>
                    </div>
                </div>
            </div>

        </div>
    </section>

Thank you anyone who can help. :)