Hi there dreamgirl2727,
From what I understand it sounds like you are unable to send messages to one specific contact but everyone else has no issue receiving your messages. I would start by checking to see if somehow the contact was added to your blocked list and verifying the information for them is correct:
If you can't send or receive messages on your iPhone, iPad, or iPod touch
- Make sure that you're using the right phone number or email address.
- Go to Settings > Messages > Blocked and make sure that they're not on your block list.
If that checks out then I would next delete the contact and re create it:
Contacts at a glance - iPhone
Delete a contact. Go to the contact’s card, then tap Edit. Scroll down, then tap Delete Contact.
Then try contacting them again. If the issue still persists, I would next delete the message thread you have with them and try one more time.
Manage conversations - iPhone
Delete a conversation. In the Messages list, swipe the conversation to the left, then tap Delete.
Thank you for using Apple Support Communities.
All the very best,
Sterling