A "Missing posts" error in Cubase means that the hardware (Apogee Ensemble) is offline or not being seen by the application. To fix this, go to the "Devices" menu and choose the last item, "Device Setup..". In the dialog box that appears choose "VST Audiobay" and make sure the device is set to "Ensemble" and not "Built-in audio".

If the Apogee Ensemble does not show up as a selection, close Cubase and power the Apogee Ensemble hardware on and off. Re-open Cubase and repeat the "Device Setup..." process above.

You may also need to remap the inputs and outputs by going to Devices/VST Connections. To remap the inputs, under the column Device Port, select Apogee Ensemble 1 and 2 for Mic inputs, and Apogee Ensemble 5 and 6 for QS8.1 inputs