ทำไมมันถึงไม่ session_start ให้ผมอะครับผมเขียนคำสั่งผิดส่วนไหนหรอครับ

<?php
$conn=odbc_connect ('ac','','');
if (!$conn)
{exit("Connection Failed: " . $conn);}
$sql="SELECT * FROM Admin WHERE Username='".$_POST["user"]."' and Password ='".$_POST["pass"]."' ";
$rs=odbc_exec($conn,$sql);
//$objDB = odbc_select_db("mydatabase");
//$objQuery = odbc_query($rs);
$objResult = odbc_fetch_array($rs);
//echo 'test=' .$rs;
//exit;
if(!$objResult)
{
	echo "Invalid Username or Password";
}
else
{
	session_start();
	$_SESSION["Login"] = "Y";
	session_write_close();

	if($objResult["Status"] =="admin")
	{
		header("location:back_shop.php");
	}
}
odbc_close($conn);
require("index.php");
?>