✅ namespaces

every time i try to use namespace and class i get

Error    CS5001    Program does not contain a static 'Main' method suitable for an entry point

Error    CS5001    Program does not contain a static 'Main' method suitable for an entry point

what am i supposed to do because

namespace helloword
{
    class HelloWorld
    {

    }
}

namespace helloword
{
    class HelloWorld
    {

    }
}

gives the same error as

namespace helloword
{
    class HelloWorld
    {
        static void Main()
        {

        }
    }
}

namespace helloword
{
    class HelloWorld
    {
        static void Main()
        {

        }
    }
}