Submenu on post

Posts aren’t hierarchical, they don’t have subpages. So there’s nothing for this widget to show.

You might try my BE Static Subpages Widget plugin. Let’s say the “Blog” page was a subpage of About Us, and you wanted the About Us section showing in the sidebar of blog posts. Create a sidebar for the blog (or use a plugin like Widget Context to limit this widget to the blog), add this widget to the sidebar, and tell it to display the subpages of the “About” page.