When you first start Cubase and you get asked if Cubasis can use your microphone you must answer "yes", otherwise you won't be able to record anything or use your connected audio interface.
If you tapped "no" by mistake, you can always change this in the settings app of your iPad. There go to Privacy -> Microphone and enable the Microphone with Cubasis.
Carlos Mendoza Rohde - Product Marketing Manager Cubase & Apps
Steinberg Media Technologies GmbH
Check out Steinberg on YouTube