Submenu don't appear

Hi everyone, on my Grav website with Twentyfifteen theme i can’t see submenu create in admin panel using “page parent”.
Any solution?

It appears that TwentyFifteen does not support any kind of child/submenus:

It has a simple loop over the top level pages. This is probably because when the theme was ported there was no submenu styling or logic in the original theme.

Compare this to Antimatter:

And you can see that this theme has much more logic including a macro that is called recursively to iterate over all pages and create a hierarchal tree that is styled by CSS into a dropdown menu system.