If you choose Doomfall’s first mode, the target opponent chooses which creature to exile. That creature isn’t targeted, so a creature with hexproof can be exiled this way.

If you choose Doomfall’s second mode, you must exile a nonland card from that player’s hand if able. If you can’t, most likely because that player’s hand contains only land cards, nothing happens. That player won’t exile a creature they control instead.