Widgets not showing on sidebars

You’ve registered your sidebars with the ids of sidebar-left and sidebar-right but you are are attempting to display them using only left and right. You need to use the correct ids. That is:

dynamic_sidebar( 'sidebar-left' );

You need that id with both dynamic_sidebar() and is_active_sidebar()

if ( is_active_sidebar( 'sidebar-left' ) ) {
  dynamic_sidebar( 'sidebar-left' );
} else { ?>
  <p> here should be a left sidebar</p><?php 
}