Spearhead, first mission, red box missing on artillery

After even struggling to activate the sequence where the Captain throws the grenade into the barn carrying the artillery emplacement, I began to search for the red box (which I assumed would've been closer to the door), yet could not find one. I went around the gun many times carefully looking at it, tried to set up the bomb in areas where it should've been, and I tried to run back and forth through the barn, try to talk to the other characters, put a grenade on it, and finally just shoot at it, yet nothing is working. Is there anyway to fix this?