In the past, I have successfully recorded a cassette into Audacity. Now, however, there is no “line in” choice in the drop down menu on Recording Device, and nothing will record. What changed? How do I solve this problem? Windows 10; Audacity 3.1.3
So that would be because most PCs no longer come with line-in ports. They have been replaced by a USB port(s) for which you will need an adapter such as a Behringer UCA222 (and similar models) and cable.
Note that most of the inexpensive USB audio adapters have only stereo output (green) and mono microphone inputs (pink). (The line-in adapter would be blue).
You may find something a little less that the Behringer, even something that includes a cable, by searching for “USB Audio Capture Card”. If you decide to go this route, check the reviews carefully and report back and let us know - most of the favorable experiences shared on the forum have been with the Behringer.