php - Not sure what I am missing?

Could someone please help, I am not sure of what I am missing here.

This is the error I get:

Notice: Undefined index: post_id in C:\xampp\htdocs\projects\charlesprater\post.php on line 6

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And this is my code as you can see below

 $db = new Database();


  $id = $_GET['post_id']; 


  $query = "SELECT * FROM posts WHERE post_id = $id";

  $posts = $db->select($query) -> fetch_assoc();

  $query = "SELECT * FROM catagory";

  $catagory = $db->select($query);

  ?>