To enable/disable the comments section on new posts & pages, check “Allow visitors to post comments on new posts” under Settings > Discussion.
Please be aware that this setting only allows the default option for new posts published, it doesn’t affect present articles at all.
The page where which you create/edit pages or posts in admin, check if you’ve got ticked to your posts/pages from the “discussion” box. If you can not see the “discussion” box, at the ‘screen alternative’ dropdown area (located near the top) there are tick-boxes to demonstrate the “Discussion” box for those posts/pages. Tick mark it. From the “discussion” box below the post/page editor, then you may allow/disallow comments for the post.
Check the theme
The problem could be related to your theme. To check if it’s the case, switch your WordPress theme. Then, re-check your comments inside posts to verify is there is the comment form.
If there is one, it means that your main theme doesn’t have a comment section block on posts & pages files. If you installed a free or paid theme, contact the developer to notify about him the problem.
If you built your own theme, then you should check the WordPress documentation to implement comments. Here is a good start.