The default setting of the Menu in the Demo you will import is for Multiple Pages. If you wish to have One Page Theme you need to set up the menu differently. But first you need to set proper anchors in start blocks of page builder template. Check the Screenshots below of how and where this need to be set up.
Once this is set go to Appearance -> Menus and add add the menu item as a custom link. Under URL write #about to point to the anchor link you created in start block. Check the Screenshot below to see how it is set up.
Settings in the Start Block
Settings in the Appearance -> Menus