Can anyone help me why this error are show?

`<?php
include_once "php/config.php";
$fname = mysqli_real_escape_string($conn, $_POST['fname']);
$lname = mysqli_real_escape_string($conn, $_POST['lname']);
$email = mysqli_real_escape_string($conn, $_POST['email']);
$password = mysqli_real_escape_string($conn, $_POST['password']);

if (!empty($fname) && !empty($lname) && !empty($email) && !empty($password)) {
    // lets users email valid or not 
    if (filter_var($email,FILTER_VALIDATE_EMAIL)) {

    }
    else{
      echo "$email + this is not a valid email";
    }
}
else{
    echo "All input fields are required";
}

?>
const form = document.querySelector("#form");
const continueBtn = form.querySelector("#conBtn");

form.addEventListener("submit", (e) => {
e.preventDefault();//preventing form from submiting
});

continueBtn.addEventListener("click", () => {
// let's start Ajax
let xhr = new XMLHttpRequest();//creating xml object
xhr.open("POST", "php/signup.php", true);
xhr.onload = () => {
if (xhr.readyState === XMLHttpRequest.DONE) {
if (xhr.status === 200) {
let data = xhr.response;
console.log(data);
}
}
}
// we have to send the form data thourgh ajax to php
let formData = new FormData(form);//creating new formdata object
xhr.send(formData);//sending the form data to php
});`

``