The Unseen Servant spell creates a "shapeless force" that is not a creature:
This spell creates an invisible, mindless, shapeless force that performs simple tasks at your command until the spell ends.
It already has been determined that it can't be shoved or grappled.
However, the Player Handbook (page 194) only mentions that creatures, objects, and locations might be targeted:
Making An Attack
Whether you’re striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.
- Choose a target. Pick a target within your attack’s range: a creature, an object, or a location.
Since the Unseen Servant is neither a creature, an object, or a location, does this mean it cannot be attacked?