Under Preferences check the Cycles section. Let me know what device is selected.
If it is not the Radeon Pro you should check that. If it is well, then the following also holds:
When you switch to Raytraced for the first time Rhino needs first to compile the kernels for it. That will take some time, but should complete in due time. In our office we have one Radeon Pro mac, and it renders once those kernels have been compiled. Unfortunately for those there isn’t any useful progress anywhere, something I haven’t been able to solve as of yet. It can take many, many minutes, so with this I can only advice patience. Once the kernels have been compiled the rendering should be able to start. If you abort the compilation by switching away to some other mode, or (force) quitting Rhino you’ll find you have to sit through the entire wait again.
But once the kernels have been successfully compiled it should not need to do that anymore the next time you switch to Raytraced.
If you don’t feel like waiting for the compile you can always select the CPU as your render device.