405 Method Not Allowed - Contact Form HTML + form.php - php

Question

I am trying to enable a simple contact form, however, every time on the button submit, I get redirected to /form.php with the error 405 Method Not Allowed in the console.
Here is the form in index.html
<div class="container text-center pt-5 pb-5" id="contact">
<h1 class="word">Contact Us</h1>
<div class="container">
<div class="row justify-content-center">
<div class="col-lg-6">
<form action="form.php" method="POST">
<div class="form-group">
<label for="name">Name</label>
<input type="text" class="form-control text-white" id="name" name="name" placeholder="Enter your name">
</div>
<div class="form-group">
<label for="email">Email</label>
<input type="email" class="form-control text-white" id="email" name="email" placeholder="Enter your email">
</div>
<div class="form-group">
<label for="message">Message</label>
<textarea class="form-control text-white" id="message" name="message" rows="3" placeholder="Enter your message"></textarea>
</div>
<button type="submit" class="btn btn-primary mt-3 btn-lg mt-4">Send Message</button>
</form>
</div>
</div>
</div>
</div>
And here is my form.php
<?php
if ($_SERVER['REQUEST_METHOD'] === 'POST') {
// Get the form data
$name = $_POST['name'];
$email = $_POST['email'];
$message = $_POST['message'];
// Set the recipient email address
$to = "hello#doctrina.ai";
// Set the email subject
$subject = "Message from Contact Form";
// Build the email content
$email_content = "Name: $name\n";
$email_content .= "Email: $email\n\n";
$email_content .= "Message:\n$message\n";
// Build the email headers
$email_headers = "From: $name <$email>";
// Send the email
mail($to, $subject, $email_content, $email_headers);
// Redirect to the thank you page
header('Location: thanks.html');
}
?>
I have thanks.html page as well. I am using Vercel for deployment. Thank you in advance for your help!