Problems setting up OpenTK in Visual Studio

Question

I am going to develop with OpenTK in Visual Studio 2019 to develop 3D graphics with WinForms and make a windows application. However, I am having great difficulty getting it set up correctly. I have installed the OpenTK with NuGet in VS and it's connected to my project. When I follow any tutorial really, I get the same error:

Any suggestions on how to solve this? I am aware of this stack post already. It did not help me.

The code I'm trying to run now is basicly only this:

using System;
using OpenTK;

namespace OpenTK_tutorial
{
    class Program
    {
        static void Main(string[] args)
        {
            GameWindow window = new GameWindow(640, 480);
            window.Run();
        }
    }
}