I have just checked installation and use on one of my test sites. All was OK.
Tested with Joomla 3.8.13, jD 3.2.62 and Jcomments 3.0.7 As it was a long time since I last checked this I did follow the documentation.So there is somewhere that has not been installed/setup correctly!!
If you would like me to look at your site to see what could be wrong then please send me a PM with super User Admin rights
just sent you a PM