“This spell can’t be countered” means it can’t be countered even by the ward keyword ability. If you cast Lithomantic Barrage targeting a creature or planeswalker with ward, you may still pay the ward cost if you want to, but Lithomantic Barrage won’t be countered if you don’t.

Spells and abilities that counter spells can still target Lithomantic Barrage. Such a spell or ability won’t counter Lithomantic Barrage, but any additional effects of that spell or ability will still happen.