I'm trying to use the Bahn.de web-site, I find the train I want and then it says it can't calculate a fare. what's with that?
There can be several reasons.
Might be the ticket is for more than 92 days in advance, but then it normally says that.
The Bahn changes their schedules slightly in June and December, and for a while, leading up to that, you can't get schedule information. But you say you can find the train, so that's probably not the case.
The Bahn only gives fares online for train for which they sell tickets online. These are generally trains with one terminus inside Germany. I.E. It won't give you a fare from Paris to Rome.
Finally (and this is what I suspect), the Bahn website does not give fares for travel entirely within a Verkehrsverbund (metro district) by transport controlled by that district. For instance, you won't find St. Goar to Koblenz by regional trains, since those trains are run by VRM, Verkehrsverbund Rhein-Mosel.
What are you looking for (start, destination, date, train #)?