Due to the specification of the driver for macOS Big Sur (macOS11), the A-88MK2 is not recognized until the MIDI application is started and put into use.
Please check if the A-88MK2 is recognized as a device when you start "MIDI Studio" in Audio/MIDI settings or a MIDI application (DAW).
If the A-88MK2 does not turn on even after trying the above, please check the following points.
- Install the driver for macOS Big Sur (macOS11) again.
- Connect the A-88MK2 to another USB port.
If the A-88MK2 is connected via a USB hub, remove the hub.
- Change the USB cable.