Search every username in your WordPress site from the author. Make the WP Search Username Widget active in your Widgets menu from your WordPress site.

Or place the below shortcode in your WordPress site page content:

[search-username]

Or place the php shortcode in your themes file with a FTP Program. You can find the normal and php shortcode in your shortcodes.txt file.

Option author page with FTP:

Only if you have not a author.php page in your themes then you can upload the author.php page from your plugin directory with a FTP program and save it up your pc directory. And upload it to your themes /wp-content/themes/your-theme-name/ directory with a FTP Program.