Lunar Force

You sacrifice Lunar Force even if the spell’s already been countered, perhaps by the ability of another Lunar Force, or if the spell can’t be countered.

You can’t choose not to sacrifice Lunar Force.

The spell will be countered even if Lunar Force can’t be sacrificed, most likely because it has already left the battlefield. Casting a spell in response to an opponent’s Lunar Force’s triggered ability is generally a bad idea.