Notice: Undefined index: session_user_start

Question

I have index.php in my project, then the error showed up while accessing the login page:

Notice: Undefined index: session_user_start in E:\laragon\www\virtualsacc\index.php on line 51

Short snippet of index.php that might relate to the error:

if (($login_user == $db_user || $login_user == $db_email) && $login_password == $db_pass){
  if ($db_accstatus == "ACTIVE"){
    session_start();
    $_SESSION['session_user_start'] = 1;
    $_SESSION['session_user_username'] = $db_user;
    $_SESSION['session_user_email'] = $db_email;
    $_SESSION['session_user_id'] = $db_id;
    $_SESSION['session_user_name'] = $db_name;
    
    header('location:banking');
  } else {
    header('location:?disabled=1');
  }


   } else {
     header('location:?error=1'); 
    }
  }
?>
<?php 
  session_start();
  if ($_SESSION['session_user_start'] == "1"){
    header('location:banking');   
  }
?>

And it's line 51 of index.php:

if ($_SESSION['session_user_start'] == "1"){

What's wrong with my code? Can someone explain and help me why isn't it working?