You may target zero, one, or two creatures. The creatures don't need to be untapped.

If you target zero creatures, all that happens is you draw a card.

If you target one creature and that target is illegal as Repel the Darkness resolves, the spell doesn't resolve. You don't draw a card.

If you target two creatures and they're both illegal as Repel the Darkness resolves, the spell doesn't resolve; you don't draw a card. If just one target is illegal, the spell does resolve; the remaining legal target becomes tapped (if it's untapped at that time) and you draw a card.