Being the first time I create a subcategory, I found some quirks while doing so:
I have set the default view to be Latest instead of Category.
While in Latest view, there are no hamburger menu where you access New Category. There are only a button for creating a new topic.
While having the Create Category window open, Parent Category select shows also subcategories.
But if you try to use a subcategory as the parent, you will have an error You can’t nest a subcategory under another. So why are these displayed in the list?
When switching to Category view, then navigating to the main category I’d like to create a subcategory in. There is no hamburger menu for this, I first have to go to the main categories overview