Seat Reservations Required, but no place to purchase

We are trying to book a train from Interlaken to Rome. At one transition, (Domodossola) the site says “You need to buy a seat reservation to board”; however, there is not a place to purchase a seat. At the next stop (Milano Centrale), it says the same thing, but with an option to purchase. Does this mean the train at the Domodossola is sold out? 

Also, it says BUS operated by Ferrovie dello stato SpA; does this mean an actual bus, or is it still a train (the other transfers say Regional Train). 

Thank you.