Visual Studio Nuget manager does not show OpenTK package

I am trying to add OpenTK as a dependency. When I open the Nuget Package Manager, the only Package source displayed is "Microsoft Visual Studio Offline Packages", so I went to Tools > Options > Nuget Package Manager > Package Sources and added the package source:

• Name: nuget.org
• Source: https://api.nuget.org/v3/index.json

I now get the following list of packages, but the OpenTK package is not there.

As a solution to installing packages that do not show up on the Nuget manager GUI, I can use the Package Manager Console which finds OpenTK without problem.

PM> Install-Package OpenTK