To use Google Messages on PC or Mac, open messages.google.com/web in a browser, then on your Android phone open Messages, tap the three-dot menu,…
The requirements are light, but all three have to be true:
The whole setup takes under a minute once you know where the button is.
This isn't a stripped-down read-only view.
You can pair more than one computer, but only one is active at a time by default.
Read the complete breakdown, fixes and what happens next.