My understanding is that this is a Rhino6 bug, and I have reported it to McNeel (about a month ago). Some users have been able to resolve this by a combination of uninstalling/reinstalling Rhino, and the plugin.
thank you Paul, the manual uninstall described on the mcNeel site worked:
-Close all Rhino applications. -Go to Windows Explorer. -Go to the C:\Program Files\Common Files\McNeel\Rhinoceros\6.0\Plug-ins. -Find the folder that contains the name of your plug-in and delete that folder.
after that i could run the installer and then I manually selected the rhi file from the rhino plugin manager .