I created a library based on .NET 4.6.2 version.
To the library, I\'ve added the EntityFramework version 6.1.3 package.
I created a model as follow
2018 update - if the accepted answer doesn't help, see this github issue on the EF6 repository.
Apparently code migration commands don't work with the new project format. In order for the migrations commands to work, you need to create a Class Library (.NET Framework) project (old standard), move all the files there, add all the needed dependencies and run the command.
EDIT: I just ran into this problem by creating a Class Library (.NET Standard) project on VisualStudio 2017 15.6.6 using EntityFramework 6.2.0. Creating an "old standard" project as explained above fixes it.