Here is how you unlock hard and challenging bounties in Division 2. You first need to find Otis Sykes who gives you the more difficult daily bounties. You can find him at the Downtown West Settlement once you finished 2 of the 4 main missions in the area.
After completing 2 settlement progress missions, return to the campus and find Otis in one of the newly established areas in the base. Talk to him and he will join your crew in the white house.
There you can also unlock the “Hard Bounties” Perk at the quartermaster.