Wyll's Reversal can be casted even if it cannot be cast legally

We were playing, when my opponent cast [[Etali, Primal Conqueror]]. The trigger goes on the stack, and me, forgetting [[Wyll's Reversal]] shouldn't work, casts it. And it resolved and copied my opponent's abilities. It is not allowable since the Reversal requires the spell or ability to have targets.