Okay, I think I know what the issue is and how to fix it.
The issue is the included ".sln" and ".csproj" files in the development example and showcase project. If you delete these files and then hit the "Build" button in the top right corner, it should successfully rebuild the project. I didn't have the correct C# libraries installed on my Windows machine that I was using for testing, but once I got everything installed and hit the "Build" button, the project compiled successfully and then I was able to enable the plugins.
I would try deleting and regenerating the ".sln" and "csproj" files and see if that fixes it. If not, can you see if the terminal/console shows any output and if it does, paste it here or take a screenshot and post that? That way I can look into what is specifically causing the issue and see if I can find a solution!