Please don't confuse Username and Nicename. Those are not the same. 

Nicenames are used for user @mentioning and in your profile URL. So again, this is the nicename, not username.

You can enable the wpDiscuz mentioning feature in the Dashboard > wpDiscuz > Settings > Subscription and User Following admin page.

For more information, visit: https://wpdiscuz.com/docs/wpdiscuz-7/plugin-settings/subscription-and-user-following/#enable-user-mentioning

Important: To display the nicenames, insert the following code in your WordPress active theme's functions.php file:

add_filter("wpdiscuz_show_nicename", "__return_true");

For guidance on adding custom code in WordPress without disrupting your site, refer to this article:: https://www.wpbeginner.com/plugins/how-to-easily-add-custom-code-in-wordpress-without-breaking-your-site/