HELP booking GOLDEN PASS with purchased Swiss Travel Pass!!!

I might be wrong, but i'm thinking the SBB site is not taking your pass because the pass is not yet active. I'm assuming since your trip is not until Sept. that you selected a Sept. start date for the pass. You won't be able to do anything with the pass until then. But i thought there was a way to input that you have a pass, without actually adding a specific pass, to purchase the seat reservation. I think someone else, Pat maybe, was asking about this not too long ago. I'd try searching the recent posts here.

Swiss Travel Pass does not include seat reservations, which must be paid separately, if that is your question.
I did find this by using the search function at the MOB Railway site, which operates the trains between Montreux and Zweisimmen.
https://mob.ch/offre/vip-seats/?lang=en

In other words, you just have to buy seat reservations (not tickets, just reservations) on the MOB website. Cost for standard seats is low, was Fr. 8 2 years ago. VIP is not available on all trains, so you might have to see if it works with your schedule!

Carrie, good point! Since the first day the Swiss Travel Pass will be active is on the Golden Pass train trip, theoretically we wouldn't be able to book the VIP seats in advance.

Carrie, we just decided to ride the regular trains on the routes, based on recommendations here. We do not need seat reservations on the regular trains, and can open windows to take pictures, jump to the other side of the train for views, and have more schedule flexibility.
Oh, how I hope we all actually get there!!!

the coaches with the "VIP" seats over the driver have been off the railway for work for quite some time now and I have not heard that they plan to return anytime soon

When they are available you get them onboard.

How to access this service? The VIP area is not available on all
trains and seats cannot be reserved in advance. To enjoy this service,
please contact the train manager. A small supplementary fee will be
charged depending on the length of your journey:

75 minutes and over: CHF 15.- 45-75 minutes: CHF 10.- 0-45 minutes:
CHF 5.-

Also on the MOB website, preview with Youtube video of the new cars that are scheduled to begin service in Dec 2022. Looks like there will no longer be those VIP seats above the driver, but a separate VIP section with extra large windows and seats that rotate 180 deg. With dual gauge bogies, it will no longer be necessary to change trains at Zweisimmen where the tracks go from meter gauge to standard gauge.
https://mob.ch/activity/goldenpass-express/?lang=en

THANK YOU ALL FOR YOUR REPLIES!!! I think the "travel gods" are showing me to not sweat the minutia and to be flexible about seating until we get there. All of your responses are thoughtful and quite helpful. I love this forum!