Yes, I have assigned it to Aux 2, and in the Optical train as both Dust cap and Lightbox.

When I control it from the Indi Control Panel / Main control and click unpark, it unparks correctly without turning on the light.

When I then click Flat Light on, I get the error message you refer to , cannot control light while cap is unparked,
but that is of course a correct error message, because the cap has to be in the park position to be able to do the flats.

I use the System Port 1a86_serial / Port usb-1a86_USB_Serial-if00-port0 / Baud Rate 19200, try to check that out in your setup.

I haven't tried in the Scheduler, busy with work unfortunately so no astro sessions right now unfortunately, so try the above
and see if you get another result this way.