So can this card work if only the light player has a unit out?

Action: Each player must sacrifice a unit, if able.

It seems like bad wording, "must" and then "if able"

If it does I understand, seems like a cheap force lightning.

There's no requirement that a player must have a unit in play to initiate the action. That's one reason why the "if able" is on the end of the effect. The "must" is a required word to compel opponents to sacrifice a unit, if they're able to. If there was no "if able", then people would ask whether the card was playable if each player didn't have a unit in play.