The option to use comments-popup.php depends on the presence of a single PHP line inside a theme’s header file. Go open the header.php of the classic theme (available in any default install of WordPress) and find:
<?php //comments_popup_script(); // off by default ?>
Many themes out there have comments-popup.php inside them but no comments_popup_script() function, making it practically useless. It’s not even available within Kubrick, WordPress’s current default theme.
Comments popup link: Displays a link to the comments popup window if comments_popup_script() is used, otherwise it displays a normal link to comments.