Counter target spell. Its controller mills two cards.
If the target spell is an illegal target when Psychic Strike tries to resolve, Psychic Strike won’t resolve and none of its effects will happen. The target’s controller won’t put any cards from the top of their library into their graveyard.
You may target a spell that can’t be countered. When Psychic Strike resolves, the target spell will be unaffected, but its controller will still put the top two cards of their library into their graveyard.