Why my iMessage needs to be enabled to send this message.

1. For iPhone to Apple Devices --> Activate iMessage
2. For sending (iPhone to non-Apple Devices )
   1. Photos/Videos/Files--> Get MMS enabled by your Carrier. Apple Inc does not provide MMS services.
   2. Texts - Get an SMS plan enabled by your carrier for the SIM/eSIM that you use on your device. Apple Inc does not provide SMS services.

To activate iMessage and FaceTime using your Phone number

First, check to know whether you have an SMS plan enabled at least for One SMS*** with your carrier. Contact your Carrier (Cellular Service Provider) the agency that issued the SIM to you

If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One SMS*** Only)

If you can't turn on or sign in to iMessage or FaceTime - Apple ...

*** 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.

You need MMS enabled by your carrier. and in your Settings. See the Pic below

Go to Settings>Messages and enable "Send as SMS". What that feature does is, in the event an iMessage fails, it allows the message to be resent as an SMS. That might make a difference in this case.