However, this option is only available if the page containing the navigation has been prepared for sorting the subordinate pages.
For subpages to be sortable, their parent page requires a custom attribute named
child_order for remembering the order of the child pages. This attribute must be of the
referencelist type, which serves to associate a CMS object with a sortable list of other CMS objects, which is exactly what is required here.
So, to make a parent's children sortable, add the
child_order reference list to the parent's object class: