I'm normally a C# developer, but I want to get Leaderboards working in my UWP app, so thought I'd look at your samples.
I've just cloned the repo and tried to rebuild the Leaderboards_2017 UWP sample using Visual Studio 2019. It asked me to retarget the project, because it wouldn't build. I retargetted to Latest (in my case), as per this screenshot.

But when I rebuild the solution, the DirectXTK project builds, but the Leaderboards_2017 project doesn't and looks like this.

What should I start looking at to get this working? Once I get it working I'll be more than happy to create a C# version and submit a PR for that, so that others don't have to go through the same pain as me.