Imessage

Hey there.

Try this link to make sure you have set up Messages correctly: Set up Messages on Mac - Apple Support. If everything is set up correctly and you still cannot send the message, use this link to troubleshoot common issues: If you can’t send or receive messages on your Mac - Apple Support

Let me know if this worked for you or not.

What happens when you try? Are you getting an error message? If so, exactly what does it say?