Hi,
I’m glad you asked instead of giving up!
This often happens when people use something like CCleaner to clean of “unnecessary” files from their computers. It can also happen with a system restore, or when upgrading from one version of Windows to another.
At any rate, the fix is to download and run Microsoft’s tool to Fix problems that block programs from being installed or removed.
Please download and run that, and then select from the list anything that looks related to Rhino 6 installation. There are several possibilities: the Rhino 6 language pack, the Rhino Installer Engine, and Rhino itself.
Once that has been cleaned up, Rhino should be able to install correctly.