It might happen if Visual Studio 2017 RC installation has been failed with a error:
Despite the error, Visual Studio will work as expected, but an official MS API (how to find all side-by-side installed VS2017 instances) will return that there is no successfully installed VS2017. More information is here https://youtrack.jetbrains.com/issue/RSRP-462225. You may run an utility mentioned in that comment to see a status of installed VS2017 RC.
To get around the problem, please run a repair action for VS2017 RC and check that the process is completed successfully.