Comments need to be enabled in Dashboard Settings and in Pages and in Posts.

(i) ... Dashboard - Settings - Discussion:

**VIP - Remember to "Save Changes."

(ii) ... And then in BOTH Pages and Posts in BOTH the Screen Options and the Quick Edit feature:- They look similar so I've included them as one:

Remember to Update.

NB. ... To remove the feature to comment from any particular page/post, just uncheck the Comments box in the Quick Edit.