I ran into this problem and all the solutions involved changing the .php.
Inadvertently, while trying to change the name of one of my pages via the "Quick Edit" option, one of the options within is to "Allow Comments". I unchecked it and was good to go.
Now why you can't allow/disallow comments through the standard edit option is beyond me, but the way I turned them off is much easier than the solutions I found myself presented with when I searched the forum. The original topic was closed two years ago and maybe that's because the fix is so easy, but if not, here y'all go!