Over the last year web browsers have been gradually phasing out support for plugins like Garmin Communicator, which allow websites to communciate directly with Garmin GPS devices. As a result we have reluctantly had to remove the direct Garmin upload and download features from plotaroute.com, as these features depended on a plugin to work.
The best way to now transfer routes between plotaroute.com and your Garmin device is to download them as GPS files and then transfer them to your device separately. If your device is recognised as a removeable drive when connected to your computer, you should be able to copy GPS files directly to your GPS device from your computer using standard file manager sofware. For other devices you will need to use third party GPS sofware like Garmin Basecamp to import and transfer the GPS files.
We have detailed instructions for downloading routes to Garmin Edge devices that also apply to some other Garmin devices.