Having Issue on Ordering CPT by Custom Field In Custom WP Query

Your query args are not quite correct. When you want to order by a custom field, you need to specify a meta_key argument, and set your orderby to the kind of ordering you need. For example:

$args = array(
    'tax_query' => array(
        array(
            'taxonomy' => 'topic',
            'field'    => 'slug',
            'terms'    => 'movie',
        ),
    ),
    'post_type' => 'essay',
    'meta_key'  => 'topic_number',
    'orderby'   => 'meta_value_num',
    'order'     => 'DESC',
);