Error - A Static readonly field cannot be assigned

You're trying to assign a value to a static readonly field (_duration or _random) outside of a variable initializer or a static constructor. This is not allowed because readonly fields can only be assigned a value either at the time of declaration or within a static constructor.

So you can use a static constructor to assigned values (_duration or _random) dynamically at runtime or remove the readonly modifier from the fields.

Static constructor:

static readonly TimeSpan _duration;
static readonly Random _random;

static Program()
{
    _duration = TimeSpan.Zero;
    _random = new Random();
}

static async Task Main(string[] args)
{
    await Start();
}

Remove the readonly:

static TimeSpan _duration;
static Random _random;

static async Task Main(string[] args)
{
    _duration = TimeSpan.Zero;
    _random = new Random();
    await Start();
}