Error di database pada program HP

Halo kawan-kawan, saya ingin bertanya.

Saya mengikuti tutorial untuk membuat form login/register menggunakan PHP dan Database, namun setelah mengikuti tutorial saya mendapat peringatan berikut. Apakah ada yang tau errornya dimana? Terimakasih.

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Kode saya :

registerdb.php

<?php
$username = " ";
$email = " ";
$errors = array();

$db = mysqli_connect('localhost', 'root', ' ', 'PRPL2021');

if (isset($_POST['register'])) {
    $username = mysqli_real_escape_string($_POST['username']);
    $email = mysqli_real_escape_string($_POST['email']);
    $password1 = mysqli_real_escape_string($_POST['password1']);
    $password2 = mysqli_real_escape_string($_POST['password2']);
}

if (empty($username)) {
    array_push($errors, "Username is required");
}
if (empty($email)) {
    array_push($errors, "Email is required");
}
if (empty($password1)) {
    array_push($errors, "Password is required");
}
if (empty($password2)) {
    array_push($errors, "Password does not match");
}

if (count($errors) == 0) {
    $password = md5($password1);
    $sql = "INSERT INTO Users (username, email, password)
    VALUES ('$username', '$email', '$password')";
    mysqli_query($db, $sql);
}

?>