I have this occur whenever I open a file that has fix sour notes enabled. This was a problem way back when this feature was added to BIAB, fix sour notes consistently generated an error. I have to go to track settings and disable this on any tracks necessary, then the error disappears and all tracks will generate.

It would be great if PG Music could fix this. I have raised it before to no avail. I fix sour notes now by changing any 7th chords to major chords where instruments, particularly pianos, play out of tune.