You may target a spell that can't be countered with Countermand. If you do, when Countermand resolves, the target spell won't be countered, but its controller will still put the top four cards of their library into their graveyard.

If the target spell is an illegal target when Countermand tries to resolve, Countermand 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.