I just had this error when updating a package in .NET Core. I have little experience with Core, and I have not seen this error in a long time. However I knew to look for the error in the "output" window in Visual Studio. What I found was 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 my problem - difference in the versions. Instead of installing version 1.1 of the package I decided to upgrade to 2.0 :)
Hope this helps someone!