Cubase

Here’s how my screens look.  I’ll start with a new Mastering file when launching Cubase.  This is a simple file with one stereo track (for a song that’s already been mixed) and the main Stereo Out.

Here’s the MixConsole for that file.  The 3 red faders on the left are the inputs.  There are only 2 inputs: LEFT and RIGHT.  Stereo In is both of them together in a stereo track.  Mono In is just the LEFT inout and Mono In 2 is just the RIGHT input.

The white fader is for the track that you are mixing.  In this case, a song that’s already mixed and is ready for mastering.

The red fader on the right is the main output.  In my experience with Cubase, this is ALWAYS labeled Stereo Out.  So I wonder if something is not set right on your interface, maybe?

When you are ready to record a mix, the File -> Export -> Audio Mixdown.  

In this window, in Channels to Export, you can specify which tracks are combined for the output.  My defaults are to export a single (stereo) channel named Stereo Out.  If you can’t select you “Main Out” from the pop-up menu next to Export Channel, then try Export Multiple Channels.

In Export Multiple Channels you should see the names of all audio tracks in your project.  Mine are called

Recorded Song, Instrument 1, Instrument 2, Instrument 3, Instrument 4

You should be able to select all that you want to include in the final mix. 

 If you are using effect channels (delay, reverb), they will show up in this middle section too.  So this could be a way to work around not having Stereo Out active.