Make it clear to the player that he must build an vehicle armed with siberite bomb so that the russian general does not fire the siberite bomb into his base.

I had built an missile silo -> he fired the missile, thought it may be related to position of missile silo, so built it at initial Powell's base -> he still fired the missile, thought he cant see my silo so fired the s-bomb right on his attack force so he does witness in action to recognize the threat -> still fired the missile, replayed from begin sparing the "Friend" and them letting him run away thinking it may be related to him -> nope, he still fired the missile, finally quit the game, opened the campaign mission to examine the script, see it checks for only vehicles armed with an siberite bomb, did so and put Macmillan as pilot into it, instead building an silo -> finally did not fire the missile.

The trigger script does not check for buildings, only vehicles. Can simply adjust it to include towers too ?

If that cannot be done then it could be in the form of an message prompt, appearing after concluding the dialog with the russian commander. But since this does not give enough time to build one, the extra time bought via dialog option then must be actually 5 minutes instead 3 minutes.

Alternately, the message prompt can appear when Stevens appears, or simply just add an extra line to speak for Stevens' dialog in this mission mentioning recommendation to build an vehicle with siberite bomb in the hopes of deterring russian attacks. That should give more than enough time for the player to research and then build the bomb armed vehicle.