index user pada session tidak terdifinisi

Mohon bantuannya gan,

Udah seharian nyari errornya dimana gan, tapi nggak dapat2

 Notice: Undefined index: USER in C:\xampp\htdocs\lrn_blog\login.php on line 6

udah di session start di init.php

<?php
require_once "core/init.php";

$error= '';

if ($_SESSION['user']){
    header('Location: index.php');
}else{

if (isset($_POST['submit'])) {

    $nama   = $_POST['nama'];
    $pass   = $_POST['password'];

    if (!empty(trim($nama)) && !empty(trim($pass)) ) {

        if (cek_data($nama, $pass)) {
            $_SESSION['user'] = $nama;
            header('Location: index.php');
            return $nama;
        }else{
            $error = "ada masalah saat login!";
        }
    }else {
        $error = "user dan password wajib diisi!";
    }
}

require_once "view/header.php";
?>

<form action="#" method="post">
    <label for="nama">Username</label> <br>
    <input type="text" name="nama" placeholder = "masukkan username"><br><br>

    <label for="konten">Password</label><br>
    <input type="password" name="password" placeholder = "masukkan password">

    <div class="error">
        <?=$error?>
    </div><br>

    <input type="submit" name="submit" value="login">

</form>

<?php
require_once "view/footer.php";
}
?>

Terima kasih telah membantu