When you edit a Section, you will see that under the Advanced Tab the right and left margins are not editable, and set to auto.
This is because, in Elementor, a section is always a complete row. If you want to restrict the content inside the section you need to go to Section > Layout and adjust the content width.
![](https://elementor.com/help/wp-content/uploads/2020/04/edit-section.png)