If you have already published a POST and comments were not enabled when it was published, you have another problem. The comments will still not be available for these.

Follow this procedure if the POST was previously published before you changed your settings.

Open your list of Posts by clicking on Posts>>All Posts To bring up a list of your posts.

Hover you cursor over the title of a post and the menu items will appear. You will choose "Quick Edit"

A detailed box will open in the listing of your post. In the Right Section of this window an Allow Comments Checkbox is present. Make sure this box is checked.

When you have checked the box, make sure you click update in the Right Corner of the box.

Next: Enable Comments on Pages>>>