If a spell or ability an opponent controls states that something happens unless you sacrifice a permanent (as Mogis, God of Slaughter does), you can't choose to sacrifice a permanent. On the other hand, if a spell or ability an opponent controls instructs you to sacrifice a permanent unless you perform an action (as Killing Wave does), you can choose whether or not to perform the action. If you don't perform the action, nothing happens, since you can't sacrifice any permanents.