Unable to apply Apogee FX Send mixer as Inputs to Cubase Group channels

When you created a Group channel in Cubase, then any FX Channels that exist will get automatically added as sends. The program does this to prevent feedback loops.

To fix this, you have to undo this by removing the new Send (select "No Bus") from the Group channel. 

https://www.steinberg.net/forums/viewtopic.php?t=56664