Last time I took the train, I ran into this same bug on the website and got the info on what went wrong from the toll-free number (evidently they can see something I can't when they try to book the trip).

If you see a trip and try to reserve your tickets with a promotion code, then it first determines if there's *any* seat on the train. That's when it offers you the option for that particular train number. THEN it looks up how many promotion-priced seats are left available on that train number. If there are zero seats left at that promotion price, then it errors with the mentioned error. You can't tell which leg of the trip is the one that's not available at that promotion and you can't book one leg at the promotion and the fully-booked leg without it. You can do this through the phone operator however (which was what I ended up doing).

So, just to give an example. Train A and Train B both have availability. If you try to book Train A and Train B for your departing and returning trains respectively and you apply a promotion code that is only allowed for the first 25 people to use that code on those trains (a smaller subset than the total number of seats available on those trains). If Train A has 5 of the 25 seats left, but Train B has 0 of the 25 seats left, then it will error the entire reservation and not inform you that it was Train B that errored nor that it was the promo code that caused the error nor offer any way to use the promo code on Train A (where availability for the lower-priced seats are still available) but offer a full priced seat for Train B.