to record a stereo track in Reaper

The default tracks are mono in Reaper. So, to record from a stereo source, as the LLM-powered search says, "create a new track and set it to record from a stereo input by selecting "Input Stereo/Input 1 + Input 2" in the track settings. Make sure to arm the track for recording and then start your audio source to capture the stereo sound."