You can cast Black Sun's Twilight without a target (presumably with X equal to 5 or more) just to return a creature card. However, if you do choose a target, and that target is illegal at the time the spell tries to resolve, the spell won't resolve and none of its effects will happen. You won't get to return a creature card to the battlefield.

The creature card you return, if any, isn't a target of Black Sun's Twilight. You choose it as Black Sun's Twilight resolves if X is 5 or more.