Warning: Cannot modify header information - headers already sent

[designavenger]

first off, i'm not sure if this is the correct subforum to submit this problem; apologies if it isn't.

The problem is:

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\include\php_layout\admin_suite\header.php:45) in C:\xampp\htdocs\admin\admin_webmasterqueries.php on line 166

admin_suite\header.php:

PHP Code:

include("../include/session.php");
    if(!$session->isAdmin()){header("Location: ../game/index.php"); exit;}
    else{ //Display everything below if user is not logged in
        include("../include/php_layout/admin_suite/header.php"); //header 


line 166 in delete function:

PHP Code:

//code executes if admin decides to delete query
    if($id == "delete" && $status == "delete") {
        
        //Added for furthur security measures - [Second check that user is authorised]
        if(!$session->isAdmin()){header("Location: ../game/index.php");}else{
        
        $query_id = $_GET['queryID']; //Gets the querie's ID
        $q = "DELETE FROM contactwebmaster_requests WHERE wmqueryID='$query_id' LIMIT 1";
        $query_delete = $database->query($q) or die("MySQL error: ".mysql_error());;
        header("Location:".$session->referrer."");}
        
    }//if ID=delete 


line 166 being:

header("Location:".$session->referrer."");}

I can't seem to figure out why its saying headers have already been sent? Any help would be seriously appreciated