Counter target creature spell. Create a 1/1 blue Spirit creature token with flying.
If Geist Snatch resolves but the creature spell can’t be countered, you’ll still put the Spirit token onto the battlefield.
If the target creature spell is an illegal target when Geist Snatch tries to resolve, Geist Snatch won’t resolve and none of its effects will happen. You won’t put a Spirit token onto the battlefield.