How to show metabox just in post.php in admin?

One simple solution is to check with the query string from the request:

if ( !empty( $_REQUEST['action'] ) && "edit" == $_REQUEST['action'] ) {
  /* proceed */
}

But it is also a good practice to use @mmm’s suggestion since you are working with meta boxes where you have the $post object in use. When composing a new post, there’s an automatically created post with a status of auto-draft, so let’s proceed the meta box coding when the status is something other than auto-draft, say publish for instance:

if ( !empty( ( $status = get_post_status( $post->ID ) ) ) && "auto-draft" !== $status ) {
  /* proceed */
}

Glad that helped.