Sidebar for category "sites"

Hi,

how can I add the sidebar to the template that is “producing” a category site (sorry, neither my WP- nor English skills are very good – so it’s hard to describe my problem).

In detail:
I created a primary menu with some static sites and two “category sites”. On one of the category site all posts of Cat_A are shown, and on the second all posts of Cat_B.
In the Pinboard-Options is the following setting: Layout > Grid Layout Dimensions > Content Columns > 3, because I want to have 2 columns for the posts and 1 for the sidebar.
Problem: there is no sidebar! Is there a posibility to add a sidebar to those category “sites” (I know, they aren’t sites but I guess you know what I mean).

Thank you for some ideas …don’t no too much about WP, so hopefully I can use your suggestions.

Bye
Karsten

PS: Is there really no search function for this support forum or didn’t I see it??? It’s no fun searching manually and might waste the time of others answering the same threats twice 🙁