Welcome to FT, beanex!

In general, you cannot book specific fare classes on united.com - it will give you the cheapest fare that matches your search parameters (based on class of service, upgrade eligibility, and promo code). If there is a Z fare offered on the route you want, and you search for Business Class fares, you will get the Z fare. If the flight shows Z fare availability and you don't get a Z fare, it means that there isn't a Z fare offered for that route (or the C or D fare is cheaper - unlikely).