Hello rihazmadr,
Thanks for posting in the Apple Support Communities. We're happy to look into this with you.
We understand that you're having issues sending texts to a specific contact from your iPhone. If you haven't done so yet, you'll want to follow the directions provided in, If you can't send or receive messages on your iPhone or iPad - Apple Support.
If you see a red exclamation point
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.
If those actions don't correct the issue, refer back to the provided article and try the troubleshooting suggestions under section: If you still need help, to see if those correct the issue you're facing.
Best regards.