Based on this previous response:
The auxiliary audio jack on your Mac isn’t the same type of thing as the one that comes out of mono cassette players. They are related, and the Mac one is made to work with one of those old cables in an audio-out capacity by default. But as an input port, it really only works with things like the older iPhone EarBuds and earphones. It won’t work with raw audio out from old audio devices.
What you need is a “usb audio adapter.” Search Amazon for that and you’ll see a variety that show up, all pretty cheap. They plug in to your USB port and then you can plug in audio cables to the other end. Then you can select that as an import source and record directly into Audacity, GarageBand, QuickTime Player, etc.
My question is: when I searched for ” usb audio adapter” it shows different ones ..some with volume control and buttons. To copy from a cassette player… do I plug in a cord from the headset into the microphone or headphone spot??