How can I trouble shoot "Your message could not be sent. #### is not registered with iMessage"? I used to be able to send txt messages from my computer through the native messages app on iOS. I recently updated and now I'm not able to send txt from my computer.

When I updated my OS, it restarted and when the screen came back on, it asked me to log into my apple id. For some reason the internet wasn't working or it wasn't connecting to the network and so I skipped putting in my appleid credentials. I looked online for some answers, but nothing satisfactory. Help?
On your phone, go to Settings -> Messages -> Text Message Forwarding and make sure that "Text Messaging Forwarding" is turned on for your system.