While I agree a save file would be more useful, I zoomed the screen in and it appears that the highlighted unit only has a machine gun in the 4th weapon's slot, which is the forward-firing only slot. You may have to turn the vehicle's facing until it can fire at the enemy unit. I've seen this before with units, they will fire their first 3 weapons at a unit in the same hex, but not the 4th weapon. I turn the vehicle facing a few different directions and at some point the 4th weapon will fire. Maybe try facing it to the bottom of the screen first? That seems to work for me most of the time.
Or move the unit one hex in any direction, face back toward the enemy unit, and then try firing
