I had seen them already. In fact, I searched and found these two topics but they didn’t help much. To deleted them I went to Edit category → Security, but it shows the message I said in my main post.
There is only one topic available in the lounge category and that is About Lounge Category which I am unable to remove since there’s no option to remove it from the topic admin settings.
This is what I see when I try to delete the category: Screenshot by Lightshot
I never posted any topic in lounge but still shows 1 topic.