Are you currently using USB multitracking mode? If so, please note that the USB 3-4 send channels are already in use.
Since these channels are occupied in multitracking mode, the toggle switch you're trying to access isn't available right now.
If you'd like to use this toggle switch again, you'll need to switch your USB mode back to stereo. Once you're out of multitracking mode and back in stereo mode, the toggle switch will be available.
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