If you cast an instant or sorcery spell, Sphinx-Bone Wand’s ability triggers and goes on the stack on top of it. The ability will resolve before the spell does.

If Sphinx-Bone Wand leaves the battlefield before its ability resolves, you won’t be able to put a charge counter on it. As a result, it won’t deal damage.

If the targeted permanent or player is an illegal target by the time the ability resolves, the ability doesn’t resolve. You won’t put a charge counter on Sphinx-Bone Wand and it won’t deal damage.