Co-Author plugin in Tortuga theme

I am using the free Tortuga theme and the Co-Author plugin. I can’t find where to edit the PHP as described here: https://vip.wordpress.com/documentation/incorporate-co-authors-plus-template-tags-into-your-theme/
Single.php does not seem to have “the_author()”in it.

For how I understand, once I get in to the correct PHP file, I would simply add a “co” to the word “author”