For me apparently it worked once I have also ran in Package Manager console :
Install-Package Microsoft.EntityFrameworkCore.Tools
Also make sure :
To have other dependencies (for example Microsoft.EntityFrameworkCore, Microsoft.EntityFrameworkCore.SqlServer, Microsoft.EntityFrameworkCore.SqlServer.Design...) referenced depending of your needs.
To select the right assembly as target for your commands in the top-right corner of the PM console (I am frequently fooled by forgetting it...)
Another problem I encountered : with the dbcontext located in a separate class library, I was encountering the following error :
Unable to find provider assembly with name
Microsoft.EntityFrameworkCore.SqlServer. Ensure the specified name is
correct and is referenced by the project.
Which I was able to fix by setting my class library as Startup project in VS (don't ask why as it seems meaningless, but it worked).
Late edit, there's something else to know : You can't run Scaffold-DbContext against a class library targetting only .Net Standard, you must also enable netcoreapp in it, or Scaffold-DbContext will complain.
To support both targets, edit the csproj to put : <TargetFrameworks>netcoreapp2.2;netstandard2.0</TargetFrameworks>
Instead of <TargetFramework>
section.
After all these you'll be able to run your Scaffold-DbContext
command line with proper arguments and connection string.
-- 2022 update --
I'm glad to see that post is still helpful as it receives some new upvotes, but instead of command-line scaffolding, there's a newer solution for the happy users of VS : you can rely on the retro-engineering feature of the extension EF Core Power tools.
I'm using it in all my new projects since a while and I find it much more powerful than raw command line, and it allows you to save your execution settings (which will avoid you to create a .bat with your custom command line).
Of course, it's your choice.