The main question is if the train is really sold out or if it’s not open for booking yet because of the track engineering. But the message given is “sold out”. And the trains before the closure going directly to Bergen the day before do show the same message.
Possible turnaround: Go by train from Oslo to Stavanger; that one is available. And from there by bus to Bergen. It’s much slower, but: the train journey is really an interesting one and even the bus journey; the bus goes on two ferries on this trip and the bus is available for 199 NOK. Of course this is a full day yourney, too, but an interesting one. There is a night train to Stavanger, too (that has availability) at 22.25 from Oslo - would mean 13.10 in Bergen by connecting bus.