If the console says that you should download the plugin despite the plugin already being installed on your Windows machine, there are a few reasons why this could be happening.
The first thing is to check (if you are using Google Chrome), that it's not a Chrome Policy Issue
Below are some steps to guide in the troubleshooting:
1. The plugin should be installed in
C:\Program Files (86)\nprtccplugin you can check whether there is a plugin installed here
2. Check the environment variables in Windows, in "Path", there should be a link to the plugin install, and only to the x86 path
3. If those fail, download Procmon, start capturing with the following filter and send us the results: