You must add a reference to assembly mscorlib, version=4.0.0

I had the same issue and adding the following package to my project.json dependencies fixed it for me:

"Microsoft.NETCore.Portable.Compatibility": "1.0.1-rc2-24027",

This package enables compatibility with mscorlib-based PCLs.

In fact, the problem is an old lib that requires an asp.net 4.0 or 4.5 vesion (less than Core).

Microsoft provides a solution for it by installing the fallowing NuGet package.

Microsoft.NETCore.Portable.Compatibility

this way you will be able to run your code with old libs.

Delete the .csproj.user as per in the Screenshot present in the solution folder. Unload the project from solution explorer. Reload the project and build it once. This worked for me.

I got this error when I created a new project using the template Class Library (.NET Standard) and ported some code into it that I wanted to re-use.

I created a new project Class Library (.NET Framework) and moved the code there instead and it worked without any issues.