No connection string named 'DefaultConnection' could be found in the application config file.

You added Connectionstring configuration to web.config file. Tried to migration, but you get error,

No connection string named 'DefaultConnection' could be found in the application config file.

And you dont know why this happening.

Its easy. You have to "Set as Start Up Project" the project containing connectionstring configuration.

And make sure that the execution of a command from Nuget console you choose the right project