What can cause header() to fail in php?

Question

I know that putting an "echo" before header() will cause it to not work, but header() is not working for me and I have not used echo

<?php
session_start();
if(isset($_GET['logout'])&&$_GET['logout']==1){
    session_unset();
    session_destroy();
}
else if(isset($_SESSION["username"])){
    header("location: prices.php");
    exit;
}
else if(isset($_POST["parse_var"])){
    require_once 'mysqli.php';
    $username = preg_replace('#[^A-Za-z0-9]#i','', $_POST['username']);
    $password = preg_replace('#[^A-Za-z0-9]#i','', $_POST['password']);
    if($username!=""&&$password!="") {
        $result=$mysqli->query("SELECT * FROM Admin WHERE username='$username' AND password='$password' LIMIT 1");
        if($result->num_rows>0){
            $row=$result->fetch_array();
            $_SESSION["id"]=$row["id"];
            $_SESSION["username"]=$manager;
            $_SESSION["password"]=$password;
            header("location: prices.php");
            exit;
        }
        else {
            $myError="Log in information was incorrect";
        }
    }
    else {
        $myError="Please enter your login information";
    }
}
?>

What did I do to cause header() to fail?