I tried to make a stereo recording from a simple UCA202 stereo USB adapter. The recording succeeded, but it was awkward and took way too much work.
In the dim distant past I was able to click inside the recording meters and they would wake up and monitor the applied audio without me going into record.
Yes, I know about the embedded volume controls. They were all the way up, but the first meter activity was when I pressed Record.
Enabling monitoring has been hidden behind the microphone icon.
Click on the microphone icon on the left end of the recording meter and the option is now there.
The exact wording depends on which version of Audacity. In 3.5.0 alpha it has been renamed as: “Enable Silent Monitoring”.