I tried updating my new MBP from macOS Sierra 10.12 to 10.12.3 using the App Store. I can see that the download of the update file finishes in App Store. I also get the prompt to restart. The machine restarts after approving it and it starts the update process. The update suddenly stops and shows an error message:
"macOS Sierra Update" could not be verified.
You can try updating again in the App Store after restarting.
I took a screenshot.
After pressing Restart, the machine restarts and macOS Sierra version is still 10.12. I tried several times to update again using the App Store but it gets stuck with the same issue. Any hints how to complete the update to 10.12.3?