When I post I don't have the option of choosing which page the post goes under

When I first started on my website, I could create a post and it would ask me what page I wanted the post to be under. Now, it isn’t giving me that option. So, when you click the blog header, it shows one post but once you click blog there are four there. How do I fix it back where I can choose the header the page goes under?

Theme name: [Enter theme name]