How can I output the slug of a comment author?

If you have the username then you should be able to get the user using the get_user_by function, which will return the user object

https://developer.wordpress.org/reference/functions/get_user_by/

$user = get_user_by(field, value);

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)