You might want to direct this question to the maintainers of the plugin, they're better informed what could be wrong. I think you're using this one: https://github.com/evothings/phonegap-estimotebeacons, is that right? In that case, Evothings forums is probably a good place to start.
My intuition: it seems like you're using an Android device. startEstimoteBeaconsDiscoveryForRegion is only available for iOS ones.