There is no direct connection from Stavanger to Voss. However, you can take the bus to Bergen(N) then take the train to Voss. Alternatively, you can drive from Stavanger to Voss in around 8 h 51 min.
How to get from Stavanger to Voss
From Stavanger sentrum Byterminalen bus to Bergen(N) then train to Voss. 7 h 17 min kr 301. From Stavanger drive to Voss. 8 h 51 min kr 929. From Stavanger sentrum Byterminalen bus to Stavanger airport, fly to Bergen airport, line 3010 bus to Bergen(N), then train to Voss. 4 h 16 min kr 967.