I cant use command INSERT INTO

I download appserv two days ago and started writing php code but ı couldn’t use INSERT INTO command. I thought the error was in the program and download xampp but i faced the same problem again. I writed a register page but ı can’t using INSERT INTO command. Not given error but not writing data to database. I dont think a problem be my connect. Here are the codes.
As I said, ı dont taken error code or notification.

<?php

require_once 'connection.php';
require_once 'register-back-end.php';


?>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="style.css">
    <title>Kayıt Ol</title>
</head>
<body>
    
    <div class="container">

        <form method="POST" action="register-back-end.php">
        <div class="input-group">
      <label>Username</label>
      <input type="text" name="username">
    </div>
    <div class="input-group">
      <label>Email</label>
      <input type="email" name="email">
    </div>
    <div class="input-group">
      <label>Password</label>
      <input type="password" name="pass">
    </div>
    <div class="input-group">
      <input type="submit" class="btn" value="POST" name="register" >
    </div>
        </form>

    </div>

</body>


</html>

connection.php

<?php

$connection = mysqli_connect("localhost","root","","account");
$db = mysqli_select_db($connection,'account')

?>

register-back-end.php

<?php

if(isset($_POST['register'])){
$isle = 'INSERT INTO users (id,username,email,pass) VALUES 
(2,"'.$_POST['username'].'","'.$_POST['email'].'","'.$_POST['pass'].'")';
}

?>

This too phpmyadmin picture