Hi again,
The templates dropdown is not available while creating / editing posts. It is only available in creating / editing pages. Here are the screenshots that will explain what you should see while creating a new post vs creating a new page.
Create a new Post:

Create a new Page:

As you see Page Attributes meta box is only available in pages while it’s not there in posts. So it’s normal not to have the templates dropdown in posts.
However to changes the layout of your posts just navigate to X > Theme Options > Layout & Design > Content Layout and select Content Left, Sidebar Right and then you’ll have the sidebar in your posts. Currently what’s happening is that you’ve selected Fullwidth in X > Theme Options > Layout & Design > Content Layout and you’re using Use Global Content Layout in X > Theme Options > Blog > LAYOUT that’s why you’re not seeing any sidebar. Just navigate to X > Theme Options > Layout & Design > Content Layout and select Content Left, Sidebar Right
This should help in displaying sidebar in posts.