Comments are enabled on some stories but not all. You’ll find at the bottom of the article if they are not enabled, it will state "Comments are not available on this story."
If comments are enabled on the article, but you do not see an option to post a comment. Please ensure you are logged into the website and logged into your commenting profile. The commenting profile is separate from the one used to login to your subscription.
If you are logged in and the option to enter a comment is still not available complete a hard refresh of your web browser.
In Chrome and Safari click the control key while clicking to reload the browser (refresh button in browser).
In Firefox hold down Ctrl and ⇧ Shift and then press R.
This action should refresh the page and commenting should appear.