Not getting both stereo channels

I’ve got my mic and my keyboard feeding to a Mackie mixer with Left and Right Main Outs feeding into a Y stereo adaptor connected to the Line Input on my iMac (desktop) computer. But Audacity is only recording one channel, not even duplicating one input to both channels, as it says on I’d like to record in stereo. Help?

Which version of Audacity are you using? The very old versions used to default to mono.

Audacity > Preferences > Devices > Recording > Channels: [X] Two (Stereo).

Does your iMac have these connections?