Calling service intent fails with "not found"
The component name is specified as the package name, then a slash, then the name of the class. So in this case, the command you want is:-

am startservice --es toggle toggle com.urbandroid.lux/.TwilightService


For future reference, and to help avoid confusion: the intent is the name of the message you're sending to start the service. The thing that you want to start is a service, not a "service intent", and the thing you found in the manifest is a service, not an intent.