1.Sign in your myGarmin account.
2.Install Garmin Communicator Plugin.(Download GCP)
3.Connect your device to the computer.
4.Your device will be scanned for available updates.
5.For 255w users, please prepare a 2G SD card before downloading the map.

Garmin Communicator Plugin is not installed.
Or the version installed is outdated.
Download and install the latest Garmin Communicator Plugin.