“Select menu for this page” missing

I have taken over a web page done by other. Somehow there was a couple of options/menus hidden on the theme which I menage to unhide by some code injection (for example the “Appearance” was not showing). Now I have come across necessity to add menus to more pages. To my surprise, after creating a new menu on the edition page of the page where I want my new menu to pop there, under the “Featured image” there is no “Select menu for this page”! The big question now is how to activate it?