Train Travel from Interlaken to Milan

Well, it sounds like you have a Eurail Global Pass. You do not need to buy a seat reservation unless you are riding one of the EC trains to Milan that you will pick up in Spiez or Brig. And then, you only need the seat reservation for the Italy portion of those trains. A simple solution would be to take Swiss trains as far as Domodossolo, then get off and switch to Italian regional train into Milan. You can ride any Swiss train with your Global Pass, thus no need to "book" these trains.

PS What is your travel day? Italian regional trains are not on the schedule yet after June 8, because of the spring schedule adjustment. So just look at the same day of the week in the next week to see the regional train schedule, as schedules do not change much, if at all. But they can't show a train on the schedule until it is official. Right now, I see regional trains every 2 hours from 12:51 until 18:51, plus some other trains that go to Porta Garibaldi station in Milan.

I am trying to book train tickets on the swiss train website but it doesn't give me an option to put in the Europe Global Pass

That statement is contradictory. You either buy a ticket, from A to B (Luzern to Milano Centrale), or you use a pass, in which case you don't need to buy a ticket, your pass is your ticket.

Sometimes with a pass you need to pay a supplement (called a "reservation fee") for some trains. This applies to Italian long distance trains, but not Swiss trains. So, with a pass, you probably (depending on which train) need to pay a train-specific reservation fee, from the Swiss border to Milan.

Rail Europe is the most expensive to buy normal tickets, but if they sold you your pass they may be the best option to pay for the supplements (reservation fees) needed on some trains.
Most passengers (locals) just buy normal "A to B" tickets, in which case they don't need to go back and pay supplements, the rail company websites are set up for these "normal" customers.

AFAIK a Global Pass is valid on all SBB (Swiss National Railways) and Trenitalia (Italian national Railway) trains. And some (bit not all) other companies in both countries.

Thank you. Yes, I meant a reservation but if we don't need to do that until we get to the country, I certainly won't. Do you think I should get reservations on all the Italy trains?

Thanks,
Rebecca

If you have a Global Pass, why lock your self in by buying reservations now. You can buy when you get there. Freccia trains require 10 EUR seat reservation, IC trains less, and regional trains none.

To be clear - it isn't a choice about buying reservations with a Rail Pass in Italy. Either you must or you can't.

If you want to ride on a high speed Freccia train you don't have a choice - you must have a reservation and when you use a pass they cost.

If you want to ride a train designated by IC or EC (InterCity or EuroCity) you don't have a choice - you must buy a reservation if using a pass and they cost.

If you want to ride a Regional train (Regionale designated R or Regionale Veloce designated RV) you do not need a reservation and reservations are not possible to buy on that type of train. Think city bus. If all the seats are taken you stand or wait for the next one. If you stand and somebody gets off, somebody else can sit down.

Wow thank you for all the replies, this has been incredibly helpful. I wish I had asked before all the research lol. Thank you so much!

Best,
Rebecca

In this particular case (Interlaken to Milan) you won't have the chance to buy your reservation at the station, as you will not be getting off the train at the Italian border. So you must buy it in advance online.
And to re-iterate: Buying a reservation as a separate purchase is not what normal travellers do.
Stations may not be set up, or the staff trained, to do this. Safer to get your reservation on the internet first.

Rail Europe is the place to buy a reservation before departure, if you wish. Otherwise, Interlaken or other Swiss train stations should be able to sell it in person.