How to add the "Comments" element to your course
To add a comment section to a lecture, open the course to display the list of modules and lectures. On the lecture you want to add the comment section to, click the 3 dots (...), then on "Edit" (number 1 in the image).
In the lecture editor, scroll down to the "Social" section on the left side of your editor, then drag and drop the "Comments" element to the desired location on your page. (number 2 in the image).
When previewing the course lecture, students will see the comment section as shown below:

Students can also reply to an existing comment by clicking on "Reply".
Students can delete their own comments by clicking on the trash can icon (see image below).
The course owner has the ability to delete all comments added to a course.
Note that if you would like to be notified whenever a new comment is added, check the "Send me an email each time a new comment is added in courses that I've created" option in the "Notification settings" section of your systeme.io account.