Apple (upper left desktop) > System Preferences > Sound. You’re looking for options and selectors to change your one connection into a Stereo Line-In. If you don’t have them, then you don’t have a Stereo Line-In and like the rest of us have to add hardware to get it.
I use the Behringer UCA-202.
It’s good quality, inexpensive, bi-directional and has zero-latency headphone monitoring should you want to try overdubbing. I have two.
There are other options, but try to stay away from adapters like the Startech ICUSBAUDIO. It stays Stereo on big letters, but only plays stereo, it won’t record it.
https://forum.audacityteam.org/t/sound-card-reviews/8375/1
Koz