You didnt say what you were recording…
Assuming vocals or guitar recording.
These are typically recorded from mono sources such as the front inputs on typical audio interfaces. You can record from the mono source to a stereo audio track and output that track to your main stereo out track. Even though it shows up visually as a single input you can still pan it and such. If you are truly recording a stereo source then you need to set that up as a stereo input buss in Cubase’s VST Connections menu.
So… provide a few more details so we can assist better but… most likely you need to record the mono source to a stereo audio track.