The device is not connected to the internet, or does not have a connection with Google servers. If it is switched on, you can send SMS commands to it from any phone - see the first two sections here. The commands sent from the website will be delivered to the device when it reconnects to the internet.
If you are testing the app, verify that the device is connected with Google servers:
- on Android 4-4.3, the 3G/4G/WiFi icon in the notification bar must be blue
- on Android 4.4 and later versions open the Quick Settings, the connection icon must be white (orange means the device is not connected, see here)
If the device is not connected with Google servers, then Cerberus can not receive the commands you send from the web dashboard. If it's connected, try to open Cerberus on the device, log in and select "Check GCM connectivity" in the configuration.
You could also try to install and configure Push Notifications Fixer (link) to have a better connection with Google servers.