Those types of messages are usually from the person you have blocked. It is very rarely an error message, but it is possible. Sprint customer service will not give you any information about the text or who it is from for security reasons, and you will not ever receive the message even if you unblock that person. The data is virtually gone forever because it had nowhere to be received.
I keep getting this same message. I also have someone blocked but not sure it's related. I called sprint and they wouldn't give up any information on who might have been sending the text. They said it could be a couple things 1) someone from a blocked number messaging you or 2) just an error message.
My phone always gets messages from "Unknown" and when I open the message, it says "Message not found". I blocked someone from calling/texting me, and I wasn't sure if I was getting the unknown message from that person since they're blocked or if it was something else.