The quickest way to update maps on your Garmin device is to via Garmin Express. Download this application on a computer, and connect a Garmin device to the computer. This will launch an installation wizard, simply follow the on screen instructions and you will have Garmin Express on your device.
This message generally displays when there is something wrong with the mapping files on the device. There are multiple fixes. First ensure that maps are enabled on your Garmin device, by going to Setting>Map & Vehicle>myMaps. In myMaps check the box next to the map you wish to load. If the error persists, then reinstall the maps, and update your device again. If the error occurred after installing a new SD card, the card itself may not be formatted correctly. Format and reinstall the SD card if this is the case, this should correct the error.