For setting a page as Onepage you have to follow following steps:
1) During creating Onepage -> Select – Template -> Single Page Template:
2) Then If you will use Elementor try to edit page – Clicking Edit with Elementor Button:
3) Into Page You have to create your page sections with a unique CSS ID:
4) After creating your onepage sections with content – you have create menu from Appearance-> Menus. And creating the menu you have to add menu items and need to set this menu as single menu in menu settins :
5) Each menu item you have to create with custom links and each custom link url will be your created section CSS ID with #.