header location not working on online godaddy hosting but working on local host

Question

the header location work perfect on local host but on godaddy hosting online not work how can i solve this problem this my login code

ob_start();
      include("includes/connect.php");
    if(isset($_POST['login'])){

$username=strip_tags(mysqli_escape_string($conn,$_POST["txt_username"]));
$pass=strip_tags(mysqli_escape_string($conn,$_POST["txt_pass"]));
if($username=="" || $pass=="")
{
echo"Enter Username And Password.";
}else{
$sql=mysqli_query($conn,"select* from tbl_user where db_username='$username' and db_pass='$pass'")or die(mysqli_error($conn));
$count = mysqli_num_rows($sql);
$res=mysqli_fetch_array($sql);
if($count==1){  
    $query=mysqli_query($conn,"update tbl_user set db_isonline='1' where db_username='$username'")or die(mysqli_error($conn));
    $user=$res['db_username'];
$_SESSION["username"]=$res['db_username'];
$_SESSION["level"]=$res['db_level'];
$_SESSION['LAST_ACTIVITY'] = time();  
header("location:main.php");exit();
}else{ echo"Username Or Password Incorrect";}
}
    }
   ob_end_flush(); 

main page code

    ob_start();
include("includes/connect.php");
 if(isset($_SESSION['username'])){ 
 $username=$_SESSION['username'];echo $_SESSION["username"];
 $query=mysqli_query($conn,"select* from tbl_user where db_username='$username'")or die(mysqli_error($conn));
 $res=mysqli_fetch_array($query);
 $fname=$res['db_fname'];
 $lname=$res['db_lname'];
 }else{
 if(!isset($_SESSION['username']) || $_SESSION['username']==""  ){
 header("location:index.php");}} 
     ob_end_flush();

and i have some php code in my header that is

     ob_start();
    session_start();

    if(empty($_SESSION['username'])) {
        echo'
        <nav class="navbar navbar-default" role = "navigation">
  <div class="container-fluid">
    <div class="navbar-header">
      <a class="navbar-brand" href="main.php"><img src="img/t-square-logo-Final.png" alt="the professionals" width="200px" style="float:left;margin-top:41px;" /></a>
    </div>
    <div class = "collapse navbar-collapse" id = "example-navbar-collapse">
         </div>
  </div></div>
</nav>
   </div>';
    }
    else{
        if($_SESSION['level']==0){
        echo'
        <nav class="navbar navbar-default" role = "navigation">
  <div class="container-fluid">
    <div class="navbar-header">
    <button type = "button" class = "navbar-toggle" 
         data-toggle = "collapse" data-target = "#example-navbar-collapse">
         <span class = "sr-only">Toggle navigation</span>
         <span class = "icon-bar"></span>
         <span class = "icon-bar"></span>
         <span class = "icon-bar"></span>
      </button>
      <a class="navbar-brand" href="main.php"><img src="img/t-square-logo-Final.png" alt="the professionals" width="200px" style="float:left;margin-top:41px;" /></a>
    </div>
    <div class = "collapse navbar-collapse" id = "example-navbar-collapse">
    <ul class="nav navbar-nav">
      <li ';?><?php if ($page_name=="search.php") 
echo"class=\"active\""; ?><?php echo'><a href="search.php">Order</a></li>
<li><a href="task/task.php?action=">Task</a></li>
      <li><a href="logout.php">Logout</a></li> 
    </ul>
  </div>
  </div></div>
</nav>
   </div>';}ob_end_flush();

i use this to print the error

error_reporting(E_ALL);

and this is the error i have but i don't know how to solve it Warning: Cannot modify header information - headers already sent by (output started at /home/mhmd2991/public_html/index.php:2) in /home/mhmd2991/public_html/index.php on line 39