seat reservations with Select pass

What error message are you seeing? I wonder whether it could be a spelling issue; the name of the station is Interlaken Ost.

No reservations are needed, or even usually possible, on Swiss trains. You only need reservations on the Venice - Milan train, and the one from Milan across the Swiss border.
And you need to ensure you spell place names correctly: Interlaken Ost. Sometimes it guesses where you mean, sometimes not.

Can you tell us how far you are getting and what error it is giving you? That helps.

Rail Europe's online shopping cart cannot currently (this month) process Italian seat reservations for passholders but they can take your order by phone at 1-800-622-8600. (If you purchased your pass through Eurail.com, then that's a separate company.) This issue also extends to reservations on the sleeper trains from Vienna to Venice, FYI.

Before you go, you can also book at www.trenitalia.com for the legs that require reservation, but not for the last legs within Switzerland (which also don't need reservations). For instance, request the 8:20 departure from Venezia S. Lucia station to Spiez (which has only one connection at Milano Centrale). After you choose your train departure, then view the pricing options in chart format, click "View other offers," then scroll through the offers and choose "Global Pass" for each person and leg. This will change the prices to 10-11 euros per person, per leg. Also enter the pass number, which is the one at the top of your pass, following CIV, and is 9 digits; or any 9 digits will work.

You can also make the reservations:

• At a staffed train station ticket window in Europe, for instance when you have your pass activated.
• At a ticket machine in an Italian station, again looking for the fare type "Global Pass."