When editing a page template using Elementor, then choosing to preview the page from the Elementor menu, you will be viewing a preview of the Elementor template, not an actual post. This means that WPDiscuz comments will not be displayed.
To actually preview the post with WPDiscuz comments working, you need to go to an actual post.
This isn't an actual question, I just wanted to post something for me to find a month from now when I have the exact problem 🙂
Please make sure the <?php comments_template() ?> WordPress template function exists when you're choosing to preview the page.
More information here: https://codex.wordpress.org/Function_Reference/comments_template