This problem comes up fairly often and I have never seen anyone post a reliable solution.
The problem is not related to Cubase (it will happen in any PC DAW) but rather the ASIO driver on PC which cannot be shared between applications - so if your DAW is using it, the Rack Pro plugin can’t use it.
In fact it may be that even if you choose a different driver for your DAW (like WASABI) then you still can’t choose ASIO as a driver from the Rack Pro plugin. I’m on Mac not PC so I can’t test that, but iirc that’s what others have said in the past.
See this thread for example: