The template you are using is not working properly for newer Rhino. This is because it targets new Net Core which had breaking changes and excluded a lot of standard library code. You are simply missing System.Drawing which is now located in an external Nuget package/library. You would need to add or reference it, I wonder why this is not fixed but I have not created plugins for a long time:
I know it might be super frustrating and hard to learn if you are a beginner. But in the very early days of Grasshopper, a Grasshopper project was completely made from scratch. Truly understanding errors and C# build configuration is a powerful skill.
In the link you find a couple of hints on what to do. I hope this helps. In general Rhino 8 is known to be a bit frustrating with C#