Update target button is disabled after schema comparison

It seems your database project has sqlcmd variables without default values.

See: http://www.andrewburrow.net/vs2012-schema-compare-buttons-disabled/

Hope this helps.

See if you have a "warning" message stating "Cannot generate deployment plan due to an internal error". If so, the Update and Generate Script buttons will be disabled.

Close Visual Studio, navigate to the folder containing the Database project and remove all (*.dbmdl) files. Then restart Visual Studio, re-run the compare schema at which point the Update button should be enabled.

Check in Error List if you have any error, I have a non recognized word in VS (but it does in SQL Server), I commented it out, re-compared and it was enabled successfully.

Workaround:

In my case. Fixing Error was not priority task. Also i was not able to find any Error in Error List.

Its just you dont have to use your "Visual Studio Database Project" in source or target. Instead of using project, create a temp DataBase using script already with you.

Select this (or these) temp database(s) in source and (or) other in target.

Button must be Enabled.

For me getting difference was far more important than fixing the issue. Hope it helps you. With some more improvisation.