You need to use new string() inside the Console.WriteLine - you can read more here: https://www.dotnetperls.com/string-constructor
Also, here is how I divide the task - into 7 parts - and then solve (draw) it piece by piece. You can read the comments in the code - it is explained there.
See the pic - I made for you :-):

Here is my answer to your question (the code):
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
class Sunlight
{
static void Main()
{
int n = int.Parse(Console.ReadLine());
Console.WriteLine("{0}*{0}",
new string('.', (((n * 3) - 1) / 2)));//PART 1 (STATIC)
for (int i = 0; i < (n - 1); i++)//PART 2 (DYNAMIC)
{
Console.WriteLine("{0}*{1}*{1}*{0}",
new string('.', i + 1),
new string('.', (((n * 3) / 2) - 2) - i));
}
for (int i = 0; i < (n / 2); i++)//PART 3 (DYNAMIC)
{
Console.WriteLine("{0}{1}{0}",
new string('.', n),
new string('*', n));
}
Console.WriteLine("{0}",
new string('*', n * 3));//PART 4 (STATIC)
for (int i = 0; i < (n / 2); i++)//PART 5 (DYNAMIC) - THE SAME AS PART 3
{
Console.WriteLine("{0}{1}{0}",
new string('.', n),
new string('*', n));
}
for (int i = 0; i < (n - 1); i++)//PART 6 (DYNAMIC)
{
Console.WriteLine("{0}*{1}*{1}*{0}",
new string('.', (n - 1) - i),
new string('.', (n / 2) + i));
}
Console.WriteLine("{0}*{0}",
new string('.', (((n * 3) - 1) / 2)));//PART 7 (STATIC) - THE SAME AS PART 1
}
}