You have two ways to add a disclaimer to your topic.
First, you can always just add a disclaimer as a standard TEXT element within your topic or article in any section. You should use this option when the Overview card is disabled in your topic or when you are creating an article.
Second, the Overview has a dedicated field for any topic guidelines including a disclaimer or other similar notes.
Information entered into guidelines field will be displayed in the Overview card of a topic (if enabled).
The disclaimer will also be visible on the Topic or Article completion certificate.