How you might seen that by default all links in MainMenu are scrolled (onepage) links, they works only on current page which makes it difficult to add subpages with separate urls. To use those typical not scrolled links you have to add extra class for chosen menu items, edit them or remember about it during adding new item(s).

NOTE! Without above class name, all external or inside links are "damaged" with "#undefined" path.

In Menu Manager  -> Menu Item -> choose "Helix Menu Options" tab from menu settings, then in Custom CSS Class please insert : no-scroll   class.

If will not help, please add "no-scroll" also in Link Type > Link Class - in selected menu item(s).