Display most viewed post from last 30 days is not working correctly

Try this:

$args = array(

    'post_type' => 'post',
    'posts_per_page' => 3,
    'meta_key' => 'post_views_count',
    'orderby' => 'meta_value_num',

    'order'=> 'ASC',
    'date_query' => array(
        array(
            'after'  => '30 days ago'
        ),
    ),
);

$query = new WP_Query( $args );

You can’t directly order by met_key’s name, you need to use it the way it is above.