@Jasonmcneish Without knowing the specific details of your environment and the person you are trying to message, it's difficult to know what the causes could be. I would recommend providing the following information at answers.microsoft.com and they can help you do some individual troubleshooting:
1. Are you trying to send a 1:1 chat or post a message in a channel?
2. Are you trying to message an internal person within your org or a guest of your tenant?
3. How are you trying to message them - ie: with a tag, opening a new chat and typing their name in, etc.
4. What happens when you do that? Does the chat window come up, can they be found, can they be found but you can't type - what's your specific experience?
Anything else that might help them understand will be helpful.