Can I connect a Yamaha Clavinova CLP-230 to a MBAir using USB cable?
You'll need to install the Yamaha USB-MIDI driver first:

https://dk.yamaha.com/da/support/updates/usb_midi_driver_for_mac.html

This driver works across all the Yamaha Clavina models with USB port for MIDI.

When you have installed the driver, reboot the Mac and insert the USB-cable from Mac to piano. Then turn on the Piano. Finally start GarageBand, and the new MIDI device should be available for use.