1.)

go to include/course-option.php file in goodlayers-lms plugin folder, at line 37, change

'supports'           => array( 'title', 'editor', 'author', 'thumbnail', 'custom-fields', 'revisions' )

to

'supports'           => array( 'title', 'editor', 'author', 'thumbnail', 'custom-fields', 'revisions', 'comments' )

2.)

in single-course.php file around line 117, add this

comments_template( '', true ); 

right after

echo '</div>'; // course-content

3.) 

Try editing the course you want to enable the comment, and select "allow comments" in "discussion tab" 

( If the discussion tab is not showing up, you may check the screen option area for this. http://www.wpbeginner.com/glossary/screen-options/ )