I found this and it worked for me.
Sonia Huelsenbeck says
APRIL 3, 2019 AT 9:53 PM
After trying all the suggested fixes, I contacted Apple. They had me do the following and it worked:
Sign out of Messages on the MacBook. We can do that by opening Messages, then going to the top left corner of the screen and click messages -> iMessages. We should be able to sign out from there .
Up next we’ll go on the iPhone to Settings -> Messages, turn off iMessage, restart the iPhone, then turn iMessage back on.
Go back on the MacBook and get signed back into the Messages app.
Go on the iPhone to Settings -> Messages -> Text Message Forwarding. We should be able to flip a switch for the Mac to allow us to send SMS and MMS messages from the MacBook, and hopefully sort out the last of it.
Enter the code that pops up on your screen.