Hi and welcome,
Make sure Cubase has enabled Mic in the System Preferences, please:
Open macOS System Preferences > Security & Privacy > Privacy > Microphone. Enable Cubase here.
Then in the Audio Connections > Inputs and Outputs, hold down Shift modifier and select the first (left) Input/Output. The 2nd (right) Input/Output will be selected automatically with the 1st one (maybe not visually, maybe it will look like an empty slot, but it should work).