At the moment you cannot intuitively specify having a Board active on subcategories. The setting itself doesn’t offer guidance or give a selection.
In order to select a subcategory, you need to enter just the slug of the subcategory (without the parent slug). I think that this is a bit unique within the Discourse UI, and is different to most other TCs. This trips up quite a few people it seems.
I can’t get them to work for my subcategories. How did you configure your subcats? Using something like category/sub_category where all lowercase regardless of display case?
and though I played with some combinations, I didn’t get the right one, which is to use the correct slug (underscore had been translated to a dash) and for category and subcategory to both be included, separately without any path in the subcat.