I exported Reaper midi notation into an xml file.
Can I import it into BIAB 2018?

Yes.

Open BiaB.

Go to File.
..then "Open Special"
......Look for "Open MusicXML file."

I imported the notation in the melody track ... I then associated with the track a vst piano with "sforzando"... I play ... I see the red note that walks but I hear nothing on this track.

Check the volume on the mixer? Make sure it's not muted?

Matt, you have the crystal ball! ah ah

No, dumb luck more often than not.