cannot start session when headers already sent in

ob_start();
session_start();
define("access", "yes");
require_once(__DIR__ . '/../engine/database/connection.php');
$us = R::load('users', 1);
if (!$us->login) {
    header("Location: /admin/install/");
}
if ($_SESSION['token'] == $us->password)
{   
    header("Location: /admin/");
}
$rand = mt_rand(0, 80000000);
?>