It’s been a while. But, in my project, I just tried importing Rhino.Geometry and it looks like that works fine.
When I look in Settings…Project…Python interpreter, I see Rhino-stubs in the list of packages. Do you see that?
Sometimes, it’s a bit mysterious how to get PyCharm to recognize imports and do the syntax highlighting. Random things which might work are File…Invalidate caches or restarting PyCharm. I don’t think I have anything else to suggest. Good luck.