has anyone you seen this one before...

it happens when my users tries to purchase a membership package. When it goes to the billing/payment page it shows this error..

 Warning: Cannot modify header information - headers already sent by (output started at /home/content/a2pnexwpnas02_data01/92/3515092/html/wp-content/themes/superlist/header.php:11) in /home/content/a2pnexwpnas02_data01/92/3515092/html/wp-includes/pluggable.php on line 1174  

im bad at this i still do understand the problem

You only shared a sliver of code and as such I think you have to trace what you are sending from the beginning. The prior I noted looked indepth plus it was easy to answer yes to your first question. As to why your setup does this, you have to trace your code to see where you tried to send a reply via PHP. I can't know this since I have no access to your project.

that is the header.php code

<?php
/**
 * The template for displaying header
 *
 * @package Superlist
 * @since Superlist 1.0.0
 */

?>
<!DOCTYPE html>
<html <?php language_attributes(); ?> class="no-js">
<head>
    <meta charset="<?php bloginfo( 'charset' ); ?>">
    <meta name="viewport" content="width=device-width">
    <link rel="profile" href="http://gmpg.org/xfn/11">
    <link rel="pingback" href="<?php bloginfo( 'pingback_url' ); ?>">

    <?php if ( is_singular() ) { wp_enqueue_script( 'comment-reply' ); } ?>

    <?php wp_head(); ?>
</head>

<body <?php body_class(); ?>>

<?php if ( class_exists( 'Inventor_Template_Loader' ) ) :   ?>
    <?php echo Inventor_Template_Loader::load( 'misc/messages' ); ?>
<?php endif; ?>

<div class="page-wrapper">
    <header class="header header-minimal">
        <?php if ( is_active_sidebar( 'header-topbar-left' ) || is_active_sidebar( 'header-topbar-right' ) ) : ?>
            <div class="header-bar">
                <div class="container">
                    <div class="header-bar-inner">
                        <?php if ( is_active_sidebar( 'header-topbar-left' ) ) : ?>
                            <div class="header-bar-left">
                                <?php dynamic_sidebar( 'header-topbar-left' ); ?>
                            </div><!-- /.header-bar-left -->
                        <?php endif; ?>

                        <?php if ( is_active_sidebar( 'header-topbar-right' ) ) : ?>
                            <div class="header-bar-right">
                                <?php dynamic_sidebar( 'header-topbar-right' ); ?>
                            </div><!-- /.header-bar-right -->
                        <?php endif; ?>
                    </div><!-- /.header-bar-inner -->
                </div><!-- /.container -->
            </div><!-- /.header-bar -->
        <?php endif; ?>

        <div class="header-wrapper affix-top">
            <div class="container">
                <div class="header-inner">
                    <div class="header-logo">
                        <a href="<?php echo esc_url( home_url( '/' ) ); ?>" rel="home">
                            <?php if ( get_theme_mod( 'superlist_logo' ) ) : ?>
                                <img src="<?php echo esc_attr( get_theme_mod( 'superlist_logo' ) ); ?>" alt="<?php bloginfo( 'name' ); ?>">
                            <?php else : ?>
                                <i class="superlist-logo"></i>
                            <?php endif; ?>

                            <?php if ( display_header_text() ) : ?>
                                <strong><?php bloginfo( 'name' ); ?></strong>
                                <span><?php bloginfo( 'description' ); ?></span>
                            <?php endif; ?>
                        </a>
                    </div><!-- /.header-logo -->

                    <div class="header-content">
                        <div class="header-bottom">
                            <?php if ( has_nav_menu( 'main' ) ) : ?>
                                <?php wp_nav_menu( array(
                                    'container_class'   => 'header-nav-container',
                                    'fallback_cb'       => '',
                                    'theme_location'    => 'main',
                                    'menu_class'        => 'header-nav-primary nav nav-pills collapse navbar-collapse',
                                ) ); ?>
                            <?php endif; ?>

                            <?php if ( get_theme_mod( 'superlist_general_action' ) ) : ?>
                                <div class="header-action">
                                    <a href="<?php echo esc_attr( get_permalink( get_theme_mod( 'superlist_general_action' ) ) ); ?>" class="header-action-inner" title="<?php echo esc_attr( get_theme_mod( 'superlist_general_action_text' ) ); ?>" data-toggle="tooltip" data-placement="left">
                                        <i class="fa fa-plus"></i>
                                    </a><!-- /.header-action-inner -->
                                </div><!-- /.header-action -->
                            <?php endif; ?>

                            <button class="navbar-toggle collapsed" type="button" data-toggle="collapse" data-target=".header-nav-primary">
                                <span class="sr-only"><?php echo esc_attr__( 'Toggle navigation', 'superlist' ); ?></span>
                                <span class="icon-bar"></span>
                                <span class="icon-bar"></span>
                                <span class="icon-bar"></span>
                            </button>
                        </div><!-- /.header-bottom -->
                    </div><!-- /.header-content -->
                </div><!-- /.header-inner -->
            </div><!-- /.container -->

            <?php if ( class_exists( 'Inventor_Post_Types' ) ) : ?>
                <div class="header-post-types">
                    <div class="container">
                        <?php $post_types = apply_filters( 'inventor_submission_allowed_listing_post_types', Inventor_Post_Types::get_listing_post_types() ); ?>
                        <?php $page_id = get_theme_mod( 'inventor_submission_create_page' ); ?>
                        <?php $page_permalink = get_permalink( $page_id ); ?>

                        <?php if ( ! empty( $post_types ) ) : ?>
                            <ul>
                                <?php foreach ( $post_types as $post_type ) : ?>
                                    <li>
                                        <a href="<?php echo esc_attr( $page_permalink ); ?>?type=<?php echo esc_attr( $post_type ); ?>">
                                            <?php echo esc_attr( get_post_type_object( $post_type )->labels->singular_name ); ?>
                                        </a>
                                    </li>
                                <?php endforeach; ?>
                            </ul>
                        <?php endif; ?>
                    </div><!-- /.container -->
                </div><!-- /.header-post-types -->
            <?php endif; ?>
        </div><!-- /.header-wrapper -->
    </header><!-- /.header -->

    <?php if ( class_exists( 'Inventor_Post_Types' ) ) : ?>
        <?php $post_types = Inventor_Post_Types::get_listing_post_types(); ?>
        <?php if ( is_singular( $post_types ) ) : ?>
            <?php get_template_part( 'templates/content-listing-banner' ); ?>

            <div class="listing-detail-menu-wrapper">
                <div class="listing-detail-menu">
                    <div class="container">
                        <ul class="nav nav-pills"></ul>
                    </div><!-- /.container -->
                </div><!-- /.listing-detail-menu -->
            </div><!-- /.listing-detail-menu-wrapper -->
        <?php endif; ?>
    <?php endif; ?>

    <div class="main">

        <?php dynamic_sidebar( 'top-fullwidth' ); ?>

        <div class="main-inner">
            <div class="container">
                <?php dynamic_sidebar( 'top' ); ?>

That's over 100 lines. As noted in the link I gave you, well first this is your code so you know it better than me, is to find if you are sending anything before headers. (quote)

No output before sending headers!
Functions that send/modify HTTP headers must be invoked before any output is made.

I do think I see echo before headers.

i didnt write the code this is a wordpress theme.. cant get any help unless i pay for another 3 months of support..

Here's the thing. Your first question is if it's been seen before. That's yes.

The next part is you run this code so you essentially own it. I don't know your site or what you did to break this. I do see an echo/print before headers. Since I don't know why your code has that, I don't know the code's intentions.

If you are paying web masters to maintain your site, then to continue you learn or pay. Here I can't edit your code because I can't know your site's needs. That's the web master's job. If this is your site, then you edit it to fix or get some staffer/volunteer to pitch in. All I can do is write looks broke to me too.

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