no Monitor output

I am brand new to Audacity. I have a iMac with the Big Sur OS (11.3). I have the audacity speaker configured for built-in output. The iMac Sound setting is for internal speakers. I can get audio sound from iMac music, sound from YouTube videos. I launch audacity configured as required for built-in output. I can record vinyl from the USB connected turntable. I click on start monitoring - I get nothing. Has anyone else seen this? or got a clue as to the problem?

Try Transport (menu) > Transport Options > Software Playthrough.

– Bill