I'm not sure what is going on. Perhaps the string part is disabled. Try this...

In your song, press the RealTracks button (the Sax icon) and choose the RealTrack Picker Dialog. This will take you to the Assign RealTrack screen. In the top left of the screen is a way to assign RTs to a particular part. It says Assign RT to this track. Pick the track you want and then choose the RT. You can do this for other tracks at the same time. After you are done, close the window. The parts you assigned should turn green. When you press Play, the RTs will generate.

In the BIAB manual, see page 195.

R