Twenty 11: sidebars & footer not displaying

by design, Twenty Eleven does not show a sidebar on single posts, and on static pages unless with the ‘Sidebar Template’;

please make sure to create a child theme to do any customisations with: https://codex.www.ads-software.com/Child_Themes

and post a link to your site to illustrate the problem.

you also might want to check your site for html validation errors https://validator.w3.org/