Sorry, having fixed the compile error just an hour earlier I jumped on this train too quickly, upon second reading I noticed that you want to use spine-monogame with a MonoGame version available for Visual Studio 2019.
Unfortunately the example project file, which has been created a long time ago, used Visual Studio 2015 and MonoGame 3.0, still listing the dependency for MonoGame 3.0 in the examples csproj file. Now Visual Studio 2019 (and IIRC also 2017) no longer supports MonoGame 3.0. Since MonoGame has been changed from a pre-installed package to being separately available (via NuGet or download), we would first need to adjust the example project accordingly, potentially fixing any problems due to breaking MonoGame changes. Before that, the example project will not load or build successfully.
