Failed to load resource 405 (Method Not Allowed)

Question

//i write a code to send message from form to my gmail but i get the following error failed to load resource the server responded with a status of 405

//(method not allowed)

HTML

<form  class="contact-form">
<input type="text" name="" id="name" /> <br />
<input type="email" name="" id="email" /> <br />
<input type="text" name="" id="subject" /> <br />
<textarea name="" id="message" cols="30" rows="10"></textarea> <br />
<input type="submit" class="submit" value="Submit" />
</form>

JS

  const contactForm = document.querySelector(".contact-form");
let fullName = document.getElementById("name");
let email = document.getElementById("email");
let subject = document.getElementById("subject");
let message = document.getElementById("message");

 contactForm.addEventListener("submit", (e) => {
   e.preventDefault();

let formData = {
  name: fullName.value,
  email: email.value,
  subject: subject.value,
  message: message.value,
};

let xhr = new XMLHttpRequest();

 xhr.open("POST", "/");
 xhr.setRequestHeader("content-type", "application/json");


 xhr.onload = function () {

// error happens here
console.log(xhr.responseText);

if (xhr.responseText == "success") {
  alert("email sent");
  fullName.value = "";
  email.value = "";
  subject.value = "";
  message.value = "";
} else {
  alert("Something went wrong!");
}

};

 xhr.send(JSON.stringify(formData));
});