Looping through WP_Post Object

Based on what you said about a post being completed, you can check if its status is completed and if it is, then point to the title:

foreach ( $lessons as $lesson ){
    if( $lesson['status'] == 'completed' ){
        echo $lesson['post']->post_title;
    }
}

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