> fluidsynth: error: Error setting the audio callback. Status=-66748
This error is printed when the call to AudioUnitSetProperty failed. According to www.osstatus.com the returned error is
kAudioComponentErr_NotPermitted. I neither know what this error means, nor how to fix it.
On the other hand, the CoreAudio driver hasn't received any real changes for over 10 years. So it may not work with MacOS 11 at all.
If you just want to get it work, you could simply specify to use portaudio or jack as audio driver.
Tom