Update Target Button is disabled in Sql Server Database Schema Comparison

This was driving me nuts, as well. I finally noticed a "warning" message stating "Cannot generate deployment plan due to an internal error". Because of this, the Update and Generate Script buttons were both disabled.

I resolved this by closing Visual Studio, navigating to the folder containing the Database project and removing all (*.dbmdl) files. I then restarted Visual Studio, re-ran the compare schema and the Update button was enabled.

The tip that helped me was to look at the error list. (View Menu, Error List). Once I cleared the errors (just the errors, the ones with the red X) I was able to compare the schema again and the Update and Generate Scripts buttons were both enabled.