The error message "Cannot output waves (Undefined external error.)" is generated by your Windows OS.

This error occurs in situations where, for example, other applications are using the audio device, or VOCALOID4 Editor cannot use the audio device because of a failure in the audio device driver.

Exit other applications and memory-resident programs that are running and then start VOCALOID4 Editor.
Also, check if the driver of your audio device is up-to-date.