SMS Messaging

I’m trying to message a specific person. I try to send the message and it keeps giving me the “Not Delivered” below the message. I’m trying to text someone through SMS, not iMessage. I tried texting a few different numbers, and they sent through immediately with no problem. I also tried to text this person on a different phone, and it sent through without an issue. I know that I don’t have a wrong number, because I have messaged them before. In fact, I was texting them just the night before.