The sidebar is missing
Go to Theme → Customize → Advanced → Sidebar and check the width for the current page type. If it is 0px, the sidebar is intentionally disabled there.
The sidebar appears only on the Blog page
That is Élan’s default configuration. The Blog sidebar starts at 260px; homepage, post, static page, category, search, archive and error-page widths start at 0px.
I enabled the sidebar on posts but the layout still looks full width
Save Theme Designer, then open an actual single post again. If the post sidebar width is set above 0px, the sidebar should appear.
I see an empty column
Check whether the sidebar is enabled for that page type but contains no visible widgets. Either add the widgets you want in Blogger Layout or return that page-type width to 0px.
The sidebar is too wide
The shared Sidebar Width is designed around a 260px default. Reduce the width or adjust the global content/sidebar gap in Theme Designer rather than forcing the sidebar with custom CSS.