You can’t cast Swift Kick unless you target both a creature you control and a creature you don’t control.

If either target is an illegal target when Swift Kick tries to resolve, neither creature will deal or be dealt damage.

If just the creature you control is an illegal target when Swift Kick tries to resolve, it won’t get +1/+0 until end of turn. If the creature you control is a legal target but the creature you don’t control isn’t, the creature you control will get +1/+0 until end of turn.