My cellular data and Wi-Fi both are on still m not able to send sms

For sending SMS you neither need Cellular data nor internet over Wi-Fi.

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

iMessage (Blue) requires internet (Either over WiFi or Cellular Service) as it is sent over internet protocol. Needs Activation on your device. Can only be sent between Apple Devices.

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. Can be sent to any Android device.