Categories box not showing on post edit pages

If you are stuck on this problem and trying to find a solution, just do one thing.

	

register_taxonomy(
  'new-category',
  'post',
  [
    'public' => false,
    'rewrite' => false,
    'show_ui' => true,
    'show_in_rest' => true,
    'hierarchical' => true,
  ]
);

When you create a new taxonomy, make sure you've set show_in_rest to true. Otherwise it will not appear in Block editor. Hope this will help you ! Please let me know if this is not working for you !