Levels of Subforums
Can I add a subforum under a subforum under a Category forum?
Forums are hierarchical they can be parent and child (subForums). They are displayed under the parent forum title. You can create the subforum as much as you want. You just need to drag and drop forum panels to set parent-child hierarchy. See the screenshot below:
More info about Category, forums and subforums can be found in this doc: https://wpforo.com/docs/root/categories-and-forums/category-vs-forums/
You are currently using the Threaded Forum Layout. The threaded layout doesn't support sub-forums yet. We'll add it in future releases. But this is possible for Extended, Simplified and Q&A layouts. If you switch your forum category to one of those layouts you'll see subForums with different list.
wpForo forum layout: https://wpforo.com/docs/root/categories-and-forums/forum-layouts/