Hi, I had this problem too, and figured it out, now it actually possible to finish the mission, there should be 2 deceased people down river that you probably didn't notice, if that doesn't work, make sure the pontoon bridge is made and all cars are out of the water, and that the main bride has already had the collapse. I was able to finish the mission. but if you really can't figure it out, type "magic" on the keyboard and then press ctrl+shift+f11, might be f10.