iMessage Issue

[facecard]

I have a Mac running High Sierra. I am trying to send a new message but when I try to send it I am getting this message "Your message could not be sent. Phone number is not registered with iMessage". I don't care about it being registered or not. Even after I add the name and number to my contacts, it still gives me that error message.

I just want to send this message to someone.

 

[pkcable]

The phone and the Mac must be on the same WiFi (or network anyways) for this to work.