Can't sign in to iMessage

Hi. I have a Macbook air 2017 and it's only a year old. I used to use iMessage all the time on it and it worked with no issues. One day, maybe a few months ago, iMessage stopped sending my messages. I couldn't send one, it would just say " [number] is not registered with iMessage," which was odd because they were, and had always been. I logged out and when I tried to sign back in, this message came up. I'm putting in the correct password and ID, but this happens every single time and it's bugging me out.

I've tried multiple different things to fix this and they haven't worked.

I really want to use iMessage again! Can somebody please help?