The package requires NuGet client version '3.6.0' or above

The package requires NuGet client version '3.6.0' or above

This is a known issue about nuget. NuGet team fixed it for Nuget 4 which comes with VS2017, but 2015 users are left behind.

Ticket: https://github.com/NuGet/Home/issues/6403

Besides, NuGet team will consider a fix. At this time there are no plans to release a new version of NuGet for VS 2015.

So if you want to use the package Microsoft.EntityFrameworkCore.SqlServer 2.1.1, you have to update your Visual Studio 2015 to Visual Studio 2017. Or if the lower version of this package is acceptable to you, you can use the version Microsoft.EntityFrameworkCore.SqlServer 2.0.3

Hope this helps.