I write romantic suspense but couldn't find the category. Where is it?

It's there — in two places since v1.8.1. When creating a book, pick the genre and then the sub-genre: "Romantic Suspense" is listed both under Suspense and under Romance, so you'll find it whichever way you think of your book. You can also always type any sub-genre by hand — the field accepts free text even if it isn't in the list.