It might be the specific train you tried to book was sold out as in Italy here will be a mandatory seat reservation, a simple check can be made by searching on the Italian operator trenitalia’s website or trainline, the EC with no seat reservation left will be shown as grey-out in the list, if you try to book from Interlaken to Domodossola most likely you will be able to get your ticket. in this case you can change to regional service at domodossola, here will be hourly regional service to Milano Centrale and Milano Porta Garibaldi. you will need to buy the ticket separately via Italian operator’s app or at the station of domodossola.