author slug in the backend

Please read the codex on this function. Outside of the loop, your second arg must be a user ID. Inside the loop, WP grabs it from the post data.

get_the_author_meta('user_nicename', $userID);

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