Nicol Bolas, the Deceiver

If the target of Nicol Bolas's second ability becomes illegal, the ability doesn't resolve and you won't draw a card. If that target is legal but can't be destroyed, most likely because it has indestructible, you still draw a card.

In a multiplayer game, each opponent in turn order makes their choice for Nicol Bolas's first ability, then all of the actions occur simultaneously. Opponents will know choices made by earlier opponents when making their choices, although a card chosen to be discarded this way isn't revealed until it's discarded.

While resolving Nicol Bolas's first ability, your opponent chooses a card to be discarded without revealing it, chooses a nonland permanent to be sacrificed, or chooses to do neither. Then that player discards that card, sacrifices that permanent, or loses 3 life. Your opponent can always choose to lose 3 life, even if they have cards to discard or nonland permanents to sacrifice.