Can I play Cannon Barrage on an enemy trying to conquer an open Battlefield?

Yes, you can play Cannon Barrage — but it won't do anything to the conquering unit.

Moving a unit to an open battlefield creates a Non-Combat Showdown (no opposing unit present), and during that showdown you're in an Open State where Reaction cards like Cannon Barrage are legal to play. However, Cannon Barrage only deals damage to "enemy units in combat," and units are only "in combat" once a Combat Showdown begins at a battlefield with opposing units (Rule 344.1, 464). In a Non-Combat Showdown there is no Combat, so the moving unit isn't in combat and takes no damage.

So: legal to cast, but it whiffs — the unit will still establish control and conquer when the showdown closes unless you remove it another way (e.g., a spell that deals damage to units at a battlefield, not just those in combat).