I don’t know what is name of this area and how can I add something as Sidebar Category or Google Adsense in this area?
Can you help me?
It’s called a ‘Sidebar’, that’s if it actually has an element there, which I don’t think Discourse does by default…
Have you seen the Discourse Sidebar Blocks Plugin?
It allows you to add a custom block of HTML to the sidebar.
Just install the plugin and create a block containing your AdSense code 
Thank you so much.
But “This plugin shows a list of blocks as a sidebar on all discovery list pages (latest, new, unread, categories but not on the single topic page”.
すべてのページで動作させるには、プラグインを修正する必要があるでしょう。
ただし、意図的に除外された可能性もあります。
あるいは、Discourse Adsense Plugin を試して、テーマの CSS を調整し、広告が適切にフローティングするようにカスタマイズしてみてください。
it does, that’s where the post timeline appears. It isn’t visible in the screenshot because the topic linked is only one post long. This is how it looks the rest of the time:
Yes, that’s because the space is already used by Discourse for a fundamental navigation element. I would hope that your users being able to navigate a topic is more important than advertising real-estate.