Message not delivered. What is wrong?

You may not have an SMS plan enabled by your carrier for the SIM that you use on your device. Apple Inc does not provide SMS services.

"Send as SMS" when enabled and when your device is not connected to the internet the iMessage will be sent automatically as an SMS assuming you have an SMS plan enabled by your carrier.

iMessage (Blue) requires internet (Either over WiFi or Cellular Service) as it is sent over internet protocol. Needs Activation on your device.

SMS (Green) does not require any internet it is sent over cellular voice protocol. Needs to be enabled by your Carrier for the SIM used by you.

It will turn from Blue to Green if you have enabled "Send as SMS". Whenever your iPhone is not connected to the internet it is sent as SMS (green) to all iPhone users.

Did you can find any solution?