Messages sending failure

Wondering if Apple support's online article (link below) can help you:

What to do: If you can't send or receive messages on your iPhone or iPad - Apple Support

Eg says:

"If you try to send a message and you see with an alert that says Not Delivered, follow these steps:

1. Check your network connection.
2. Tap , then tap Try Again.
3. If you still can’t send the message, tap , then tap Send as Text Message. Messaging rates may apply. Contact your carrier for more information.

Learn what to do if you still receive a Not Delivered alert after you try to send your message as a text message.

You can set up Messages so that it automatically

tries to send messages as an SMS when iMessage isn't available.

Go to Settings > Messages, and turn on Send as SMS".