header overwrites post

This must be a really dumb question, but the solution seems to be out of my grasp. I am developing a test site with Astra theme, mainly to learn, and in fact for the moment I am using the infinity free hosting. The problem is that when I add a post, the post is displayed on its page, but is overwritten by the page header, as you can see. I would like to keep the header, but how can the post be shown correctly?

I apologize for the naivete. Just trying to learn

The page I need help with: [log in to see the link]