Why custom taxomony not showing when create a post?

It looks like you’re using the new Block Editor (Gutenberg).

In such case, you should know that this editor is based on REST. So if you want to see your custom taxonomy in this editor, you have to make it visible in rest.

You should set show_in_rest parameter to true.

$args = array( 
    'labels' => $labels,
    'public' => true,
    'show_in_nav_menus' => true,
    'show_ui' => true,
    'show_tagcloud' => false,
    'show_admin_column' => true,
    'hierarchical' => true,
    'with_front'=> true,
    'query_var' => true,
    'has_archive' => true,
    'rewrite' => array( 'slug' => 'state' ),
    'show_in_rest' => true
);