1st post… WELCOME!
Just guessing as you have not listed any system or software details. You can list your stuff in your forum signature.
So, some thoughts…
Are you sure the R24 is connected to a proper working USB2 or 3 port (not sure if it will work on a USB3)?
Is the R24 powered up and recognized by the computer before starting Cubase?
Have you installed the latest R24 driver?
Have you followed these guidelines?
Regards 