Fatal error: Can't use function return value in write context in

I'm getting this error and I can't make head or tail of it.

The exact error message is:

function kdrusha_theme_create_page() {

require_once(get_template_directory().= '/inc/pages/kdrusha-settings.php');

}

add_menu_page("KD Rusha Options", 'KD Rusha', 'manage_options', 'kdrusha-options', 'kdrusha_theme_create_page','',99);

via Chebli Mohamed