Since it's been a year with no solution to this issue I am coming around to the realization that maybe GarageBand is not the best DAW to interface with B-I-A-B?? I don't know. If there is a DAW that is optimal for use such as Logic or Logic pro. I'm more into pure playing than tech but if it means I can get the B-I-A-B plugin to work I will be glad to.

Currently my workflow is: I get my BIAB tracks and project finished then, I I take each individual BIAB track ...bass, piano, drums, guitar and I creat AIFF files for each individually. then I import them one at a time into GarageBand. This is the only way I know how to be able to maintain control of the levels and such for the individual BIAB tracks.

Sure would save time if I could create the BIAB tracks within GarageBand from the start!

If there is an answer out there to the question "what DAW works best for BIAB plugin for mac?"... I sure would like to start learning on it so maybe by the time BIAB 2022 becomes available I can hit the ground running!
ANYBODY???