Magister comes with two blogposts options: fullwidth or sidebar.
To change these blocks, go to Divi | Theme Builder click on the edit buttons in the corresponding “Custom Body” section:
As you can see, the “Blog Post with Sidebar” identifier appears in red. This is because there are no sections associated with this block (all posts are preconfigured to be displayed as fullwidth).
If you prefer to use posts with sidebar, click the settings button in that section and then choose where it should be displayed, in Template Settings window:
Don’t forget to click on “Save” button before leaving, to aplly the changes you made.