Page Comments option not showing on edit page (Screen option included))

Hi,
I can’t enable comments on my pages … why ?
Well, i don’t see “Page Comments” nor its probable translation in french (“discussions”) in my [edit page] pages.
It is in [edit post] pages. ??
I don’t see it even in the screen option up there.
Strange : in my network I have other sites where the “Discussions” option appears … but here, it doesn’t

Is there an option that I may have missed somewhere ?

Thanks for any clue

using Wp WordPress 3.9.2 as a network
Theme : Sterling 2.2.1
I tried to disable my plugins … no effect