ACF Select Main Menu for Specific Page

Hi All, I have a client that would like to have a select field on the page using a custom template – allowing them to select the main menu to be displayed. I’m using Advanced Custom Fields plugin for the select field. I have created a page template based on the template-builder.php. The new template calls a custom header file (copy of header.php) that finally calls “helper-custom-menu” (instead of “helper-main-menu”). I think I found the area where I need to change the code to make this all work, but it surely does NOT work. Does anyone have any suggestions on what needs to be changed for this to work correctly? Here’s the code section I changed:

                        /*
                        *   display the main navigation menu
                        *   modify the output in your wordpress admin backend at appearance->menus
                        */
                            if( $headerS['bottom_menu'] )
                            {
                                $output .= '</div>';
                                $output .= '</div>';

                                if( ! empty( $headerS['header_menu_above'] ) )
                                {
                                    $avia_config['temp_logo_container'] = "<div class='av-section-bottom-logo header_color'>{$output}</div>";
                                    $output = '';
                                }

                                $output .= "<div id='header_main_alternate' class='container_wrap'>";
                                $output .= "<div class='container'>";
                            }

                            $avia_theme_location = 'avia';
                            $avia_menu_class = $avia_theme_location . '-menu';
                            $select_menu = the_field( 'select_wp_menu' )

                            $main_nav = "<nav class='main_menu' data-selectname='" . __( 'Select a page', 'avia_framework' ) . "' " . avia_markup_helper( array( 'context' => 'nav', 'echo' => false ) ) . '>';

                            $args = array(
                                        'theme_location'    => $avia_theme_location,
                                        'menu'              => $select_menu,
                                        'menu_class'        => 'menu av-main-nav',
                                        'container_class'   => $avia_menu_class.' av-main-nav-wrap'.$icon_beside,
                                        'items_wrap'        => '<ul role="menu" class="%2$s" id="%1$s">%3$s</ul>',
                                        'fallback_cb'       => 'avia_fallback_menu',
                                        'echo'              =>  false,
                                        'walker'            => new avia_responsive_mega_menu()
                                    );

                                $wp_main_nav = wp_nav_menu( $args );
                                $main_nav .= $wp_main_nav;