I played this game a lot back in the day. The GOG version was working until we got to The Bridge. All tasks were fulfilled, including destruction of the enemy tanks, but notwithstanding that, every time I tried it again, the narrator tells Powell "You're too late, get out of the tower." SO I tried getting out of the tower alive and succeeded, but you can't get to the tank and there seems to be nowhere to go. Never had this happen in the original version of the game, so must be a glitch. UNfortunately this glitch appears to mean that it's not possible to finish the game.

Does your allied tank keep getting damaged? If so, there are still enemy tanks on the map. You have to find another vantage point from where you can call in an airstrike on them. Try the building to the right of the bridge, you can call the plane by looking out from the window upstairs.