UR242 left channel "inactive"

When I start a mastering project through Cubase hub, in Device Setup, Yamaha Steinberg USB ASIO, the Output Mix Left channel is labelled “inactive”. I cannot find any way to make it active. Can anyone tell me why?
I’ve tried resetting the ports.
This is only in mastering. It is “active” in recording mode.
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Are both outputs assigned to the outputs in VST-Connections (F4)?

Thanks, that was it. The left channel was set to FC Bus L, and the right channel was set to Mix R. I don’t know why they were set like that, though.