There is no such thing as a “one way Saver Day Pass ticket”. A Saver Day Pass, is a Pass. Good for all public transit in Switzerland, so as such does not have a route or a direction.
There are indeed recurring issues with buying tickets and passes on SBB from the US. the core issue is that the US is behind Europe when it comes to credit card security standards. A lot of on line vendors do permit purchases form the US in spite of that, but SBB is rather picky here.
However it is possible that there were indeed just some temporary technical problems and that you should try again later. And for buying Saver Day Passes you best go here: https://sbb-tickets.cc/en/offers/saver-day-pass
There is no need to go via the timetable search.