The project that creates a class library cannot be directly launched

I created a project in a unity, created a c # script, the visual studio opens, I pressed start and an error appears. The project that creates a class library cannot be directly launched
what did I do wrong? how to fix?

 

What "start" button are you referring to? If you want to play your project in the editor you press the "Play" button at the top.

 

start in visual studio. i did new unity project, created default c# script and tried to compile that script in visual studio

 

You don't run Unity projects from VS. Unity isn't just a simple library you add to a VS project. If you want to build and run your project, you do it from the Unity Editor.

 

Since @Joe-Censored touched on the way to make it work I'll touch on the technical explanation. Unity ships with its own compiler that is mostly equivalent to C# 7.3 and it's own framework that is mostly equivalent to .NET 4.6. Unity's runtime libraries expect your scripts to be compiled and implemented using these.

If I remember correctly there used to be a way (and maybe it still exists but my searches turned up nothing) to start your game from within Visual Studio without having to switch focus to Unity but I don't believe that functionality exists now.