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