The sidebar is missing on one page type
Open Layout → Theme Options and check the corresponding setting. For example, an individual post uses sidebar-post, while the chronological blog listing uses sidebar-blog.
The sidebar appears where you do not want it
Set that page-type option to no. Avellin reads the options independently, so changing the Blog sidebar does not automatically change posts or pages.
The sidebar is too narrow or wide
Visibility is controlled by Theme Options; width is controlled by Theme Designer. Adjust the Sidebar Width control rather than trying to encode a width into the yes/no widget.
The post grid changes when the sidebar is active
This is intentional. On desktop, Avellin reduces listing grids to two columns beside an active sidebar so cards remain readable.
Mobile behavior
At smaller widths the layout becomes single-column and the sidebar moves beneath the main content instead of squeezing both columns together.