I just had this error when updating a package in .NET Core. I always forget what this error means as there is a long time between I see it. Looking at the "output" window in Visual Studio, you will see something like the following:
Package Microsoft.AspNetCore.Mvc.Core 2.0.0 is not compatible with netcoreapp1.1 (.NETCoreApp,Version=v1.1). Package Microsoft.AspNetCore.Mvc.Core 2.0.0 supports: netstandard2.0 (.NETStandard,Version=v2.0)
Package restore failed. Rolling back package changes for 'Asp.net core'.
Which makes a great sense. Because: I was using .NET Core version 1.1 instead of 2 and then I tried to install a Core 2 package. So there is the problem: a difference in the versions. Instead of installing version 1.1 of the package I decided to upgrade to .Net Core 2.0. Whether you want to upgrade your .net core version or install older packages is up to you.
I hope this helps someone, please let me know in the comments down below if it did!