help sidebar – moves to bottom

there is something seriously wrong with the comment form
have you modified a template file?
have you tried deactivating all plugins?

I have no active plugin and I don’t think I have embedded template file (which template file do you mean…)

thanks ??

how did you integrate the sidebar into single.php of Twenty Eleven?

a:
please don’t edit Twenty Eleven directly – but create a child theme https://codex.www.ads-software.com/Child_Themes to make the edit in there.
it is important to have an unedited default theme (Twenty Eleven) in case of problems. also, any customisations will be lost with the next upgrade of your wordpress version.

b:
to have a sidebar in single posts, you would ‘normally’ need to make adjustments to functions.php or at least to style.css;
for instance according to following instructions