What is the difference between macOS Sierra 10.12.2 Update and macOS Sierra 10.12.2 Combo Update?
I believe that the "normal" update only has the changes from 10.12.1 to 10.12.2; therefore, you need to have 10.12.1 already installed.

The combo one, OTOH, has all the changes from the last major release (10.12.0).