Board Thread:Support Requests - Designing Your Wiki/@comment-24751497-20170617143919/@comment-168424-20170620001826

You are only allowed to modify the sidebar on the main page, not on other pages.

To have stuff show up in the main page sidebar, put it in between these tags:

The left side bulk of the main page should go inside these tags:

Otherwise, there is no "location" for right sidebar content, since you're not allowed to modify it wiki-wide. If you want to modify it for just your user, you would need personal JS (Special:MyPage/wikia.js; or CSS if the changes are cosmetic: Special:MyPage/wikia.css).