A Basic Page is already added to your Structure when you receive your blank website, so do not try to add a Basic Page from the Structure tab of your Admin/User Bar.
To create a basic page, on your Admin Bar, go to Content, then Add content. Click on Basic Page, then add a title to your page, e.g., About Us, which will become part of the URL for this page.
See Content Formatting that is located in another topic area. Content Formatting will provide more information about the placing of text and images in the body of your content.