Issue building solution for Visual Studio

Hello!

I've been trying to build a UWP app for the HoloLens, and I needed to import some classes from a regular project I had before that was using .NET Standard 2.0. I put these classes into a DLL and imported it to Unity and added it as a reference to the script that I built, as well as the Visual Studio Project I have been using.

I had a couple problems before, but they were mostly solved by updating from Unity 2017 to the 2018.1.5 version. Now in this version, I have the Unity project API Compatibility Level set to .NET Standard 2.0, but when I add the script to the Game Object it shows me this error: "The type 'ValueType' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'."

Is there anything else that needs to be done in order to reference/be able to use .NET Standard 2.0 in my project? Thanks in advance.