Comments are enabled on theme’s posts and pages by default. You can change the default for new posts or pages, as well as enable/disable comments on posts or pages you’ve already published.
To modify if comments are displayed or not at all, you’ll need to go to
Settings → Discussion screen. There, you can choose to allow comments or not.
Missing Discussion Module
The edit screen on your WordPress sites highly customizable, you can move them around to top, bottom, or sidebar as per your preference and also enable/disable them. So this module may not be always visible on post edit screen depending on the settings.
You can reveal it by clicking on Screen Options on the top right, and checking the box that says Discussion.
To change the comment settings for any post/page, first locate the post you want to edit—then locate the discussion module on edit screen and either check or uncheck the options it offers: