Your project is not referencing the ".NETFramework,Version=v4.5" framework.

I found that a rogue project.lock.json was causing this issue for me. Once I deleted the file, the problem went away.

Here is some documentation on project.lock.json.

Some further advice would be to ensure that project.lock.json is ignored in your git ignore file, and to try a full git clean. Make sure you understand the implications of a Git clean before you do that though.

I had the same problem - this occurs still using Visual Studio 2017.3, which uses .csproj files instead of project.json. Interestingly, the error message still contains the text "project.json".

It appears the cause of this issue is a lock file or obj\project.assets.json file (depending on your version of VS) from a previous build, which is not removed during a clean, as described here.

Manually deleting the /obj directory is a quick workaround.

If you have a "new" project (eg .NET Core or netstandard project created in VS 2017) and an "old" project in the same directory (eg .NET 4.6 project created in VS 2015), it appears they will continually fight because they both use the ./obj dir in different ways. More info here..

The msbuild workaround is to make one of your projects use a different obj dir. I added this to my "old" csproj:

<PropertyGroup>
  <!-- Needed due to old project and new project in same directory: https://github.com/NuGet/Home/issues/5126 -->
  <BaseIntermediateOutputPath>obj_netfx\</BaseIntermediateOutputPath>
</PropertyGroup>

I had this same problem, and I ended up having to not only delete the contents of bin and obj folders, but also the .vs directory for the solution.