Simple Hello Program is not passing test on my code wars task

My program is suppose to pass a series of tests but it fails. The program is make a simple function called greet that returns the most-famous "Hello world!".

Here is my code

using System;
using NUnit.Framework;
using Kata;

namespace Kata.greet

{

static void greet()

{
return "Hello world!";
}

}

The Test it is using is below

using System;
using NUnit.Framework;

[TestFixture]
public class GreetTest
{
    [Test]
    public void ShouldReturnHelloWorld()
    {
        Assert.AreEqual("Hello world!", Kata.greet());
    }
}
    

Please can someone help, I am trying so hard but getting nowhere. Where on earth am I going wrong?

When I run it now it says >

src/Solution.cs(8,17): error CS0116: A namespace cannot directly contain members such as fields, methods or statements