There may be several reasons why you are experiencing a communication error while transferring MIDI files from your Yamaha P-115 to your PC. It is recommended to check if the USB cable you are using is functioning properly. Additionally, ensure that you have the latest drivers installed for your Yamaha P-115 and that your PC is compatible with the MIDI transfer process. Restarting both devices and trying again is also worth a shot. If the issue persists, contacting Yamaha customer support for further assistance would be advisable.