MySQL Error, Does Not Save Information In Database

Question

This is what that comes up, after I click submit button:

Warning: mysqli::__construct(): The server requested authentication method unknown to the client [caching_sha2_password] in C:\xampp\htdocs\tonevre\insert.php on line 14 Warning: mysqli::__construct(): (HY000/2054): The server requested authentication method unknown to the client in C:\xampp\htdocs\tonevre\insert.php on line 14 Connect Error(2054)The server requested authentication method unknown to the client

This is my INSERT.PHP code:

<?php
$firstName = $_POST['firstName'];
$lastName = $_POST['lastName'];
$email = $_POST['email'];
$password = $_POST['password'];

if (!empty($firstName) || !empty($lastName) || !empty($email) || !empty($password)){
    $host = "localhost";
    $dbUsername = "root";
    $dbPassword = "";
    $dbname = "database";

    //create connection
    $conn = new mysqli($host, $dbUsername, $dbPassword, $dbname);

    if (mysqli_connect_error()) {
        die('Connect Error('. mysqli_connect_errno().')'. mysqli_connect_error());
    } else {
        $SELECT = "SELECT email From database Where email = ? Limit 1";
        $INSERT = "INSERT Into database (firstName, lastName, email, password) values(?, ?, ?, ?)";

        // Prepare statement
        $stmt = $conn->prepare($SELECT);
        $stmt->bind_param("s", $email);
        $stmt->execute();
        $stmt->bind_result($email);
        $stmt->store_result();
        $rnum = $stmt->num_rows;

        if ($rnum==0) {
            $stmt->close();

            $stmt = $conn->prepare($INSERT);
            $stmt->bind_param("ssss", $firstName, $lastName, $email, $password);
            $stmt->execute();
            echo "New record inserted successfully";
        } else {
            echo "Someone already register using this email";
        }
        $stmt->close();
        $conn->close();
    }
} else {
    echo "All field are required";
    die();
}
?>