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.

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.