Hey, I am trying to create a menu for a single Blog Post only.
Custom menus apply to sites not to posts or pages.
You begin by assigning categories to your posts.
This is a one or the other situation. You have two choices on any blog
(1) All posts on the front page.
(2) Or a static front page http://en.support.wordpress.com/pages/front-page/ and all posts on a different page.
By default the front page of a blog displays all published posts in reverse chronological order with the most recently published post on top. When we publish a post on WordPress software the software automatically creates the same entry in the Archives, and on the dynamic (automatically updating) Categories and Tags pages in accord with the Categories and Tags we assign to that post.
So when we create custom menu and add the dynamic (automatically updating) category pages to that custom menu we create the appearance (not the reality) of posting to more than one page.
You need to be aware that:
1. There are differences between posts and pages described here
2. There are differences between static pages we bloggers create https://en.support.wordpress.com/pages/ that will not automatically update, and dynamic category pages created by the software when you publish a post which will automatically update https://en.support.wordpress.com/category-pages/.
3. We organize posts by category assignment prior to publication. http://en.support.wordpress.com/posts/categories/ and when we publish posts the dynamic category pages they automatically display on are determined by the categories we assign to them.
So the process you need to use is:
a. Assign Categories to your Posts. https://en.support.wordpress.com/posts/categories
b. Create a custom menu https://en.support.wordpress.com/menus/#1-create-a-custom-menu
c. Add only the dynamic category page links into the custom menu that you want to appear in that menu https://en.support.wordpress.com/menus/view-all/#adding-category-pages and arrange them as you wish them to appear https://en.support.wordpress.com/menus/view-all/#changing-the-order-creating-sub-menus
d. Create pages and sub-pages for static content, not for posts https://en.support.wordpress.com/pages/
e. Add any custom links and other items like static pages and sub-pages into your custom menu
and arrange them as you wish them to appear https://en.support.wordpress.com/menus/view-all/#changing-the-order-creating-sub-menus
Please see https://en.support.wordpress.com/customizer/