This can occur if your solution contains PCL and/or older .NET Standard projects together with newer .NET Standard projects. PackageTargetFallback has been deprecated and replaced with AssetTargetFallback resulting in a nuget restore failure.
In your .csproj you should see this line:
<PackageTargetFallback>$(PackageTargetFallback);portable-net45+win8+wp8+wpa81;</PackageTargetFallback>
You can likely just delete it but if you are still running into build failures, you can also rename it as referenced below:
<AssetTargetFallback>$(AssetTargetFallback);portable-net45+win8+wp8+wpa81;</AssetTargetFallback>