The order of topics (blocks) and lessons is set on a separate course sorting screen, rather than by dragging and dropping on the main page.
To change the order of lessons and blocks, open the "Courses" section, select the desired course, and click "Sort" on its main page or in the "Lessons" tab. This will open sorting mode: up and down arrows will appear for each block and lesson—you can adjust the order. To exit the mode, click the off button in the yellow box; this will return you to the list of lessons.
How to create blocks - in How to break a course into blocks? .