Based on this previous answer:
Extra audio connector on your Mac is not the same type of thing as the one that comes out of mono cassette players. They are related, and the Mac is designed to work with one of the old cables in a standard audio output capacity. But as a gateway, it really only works with things like the older iPhone EarBuds and headphones. It will not work with raw audio output from old audio devices.
What you need is a "USB audio adapter." Search Amazon for it and you'll see a selection that pops up, all pretty cheap. They connect to the USB port, and then you can connect audio cables to the other end. Then you can choose it as an import source and record directly in Audacity, GarageBand, QuickTime Player, etc.
My question is: when I searched for "usb audio adapter", it shows different. Someone with volume control and buttons. To copy from a cassette player … do I plug a cord from the headset into the microphone or headset slot?