If the targeted creature loses all its -1/-1 counters by the time the ability resolves, the ability won’t resolve because its only target is illegal. The creature won’t be destroyed.

This ability is mandatory. If you’re the only player who controls a creature with a -1/-1 counter on it as your upkeep begins, you must target one of your own creatures.

You can’t target a creature unless it already has a -1/-1 counter on it as your upkeep begins. Putting a -1/-1 counter on a creature during your upkeep won’t let Crumbling Ashes destroy it that turn.