Hmm, I'm not really finding much when searching around for others with similar issues.. So I'm gonna be kinda winging this
Are you around or connected with someone that you can test this with now?
What if you turn that switch ON, send a couple of texts, then turn it off and try again?
What if you then turn the switch off and do a force reset of your phone. The following link will have instructions for how to do the hard reset
https://support.apple.com/en-us/HT201412
Once the phone is back up, try sending message again and see if it's working correctly.
If still not, the next thing I'd try is turning off the iMessage switch. Wait a few minutes, then turn iMessage back on and let it activate. Try texting again.
If none of this works, I'm not sure what else to try.. maybe someone else will post in with suggestions