This is not an expert opinion.
It's to do with your sound card. Ideally it would provide StereoMix under Recording Devices; failing that, RecPlayback.
Right-click on the speaker icon in the system tray and select Recording Devices, then right-click on a blank space and click Show Disabled/Disconnected Devices.
Heaven knows why I've got all this carp in there :-/
Anyway, if StereoMix is there, left-click on that and set it as Default (at the bottom). Failing that, RecPlayback.
With RecPlayback, the disadvantage is that if you adjust the volume on the PC it will affect the volume on the recording. Therefore, on my Dell laptop that has IDT soundcard without StereoMix, I use an external speaker that plugs into the headphone socket, then I can adjust the volume without affecting the recording.
Under Audio in the Debut Options, tell it to record from speakers. But I get in such a muddle with it I sometimes tick Mike and Speakers ;-)
If you get that set up properly, Debut should be able to choose the correct audio source.
Without knowing anything about your hardware, that's the best I can do.