You must republish the site after enabling the comments. Check also that the “Comments” element in the footer is turned on in the article’s settings.
To add comments your site visitor must be logged in to a Facebook account. Make also sure that you have enabled Facebook’s optional cookies from your cookie settings which can be found in the Facebook settings menu when logged in. Otherwise, the comment box won’t show up.
You can manage your cookie settings on desktop Facebook via the settings menu found in the top right corner of the viewport (Settings & Privacy -> Settings -> Cookies).
"We will no longer support the ‘Like’ and ‘Comment’ Social Plugins for European Region users, unless they are both 1) Logged into their Facebook account, and 2) have provided consent to the “App and Website Cookies” control."