Ok, really not intuitive. I typed and “created” the category, and it worked. But it still does not appear as expected.
I did a fresh Discourse install to test, set up both components:
(D.I.Y. / diy is the category I want to enable KB for, and it has a specific top link created with Custom top navigation links component)
But the D.I.Y. category appears with normal view:
URL shows at the end: /c/diy?kb=active