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. Whether you want to upgrade your .net core version or install older packages is up to you.
Hope this helps someone, please let me know in the comments down below if it did!