Why i can't insert values in private arrays?

using System.Drawing;
using System.Runtime.CompilerServices;

namespace _2_Print_the_sum_of_each_raw_from_3d_matrix
{
    internal class Program
    {
        private static readonly Random RandomGenerator = new Random();
        private List<int> ArrayOf3xMatrix = new List<int>();


        static void Main(string[] args)
        {
            ArrayOf3xMatrix = FillMatrix(3);
        }

        private static List<int> FillMatrix(int size)
        {
            List<int> matrix = new List<int>();
            for (int i = 0; i < size * size * size; i++)
            {
                matrix.Add(GenerateRandomNumber());
            }
            return matrix;
        }
        private static void PrintRandomMatrix(int size)
        {
            for (int j = 0; j < size; j++)
            {
                for (int i = 0; i < size; i++)
                {
                    
                }
                Console.WriteLine();
            }
        }
        private static int GenerateRandomNumber()
        {
            return RandomGenerator.Next(0, 100);
        }

    }
}

using System.Drawing;
using System.Runtime.CompilerServices;

namespace _2_Print_the_sum_of_each_raw_from_3d_matrix
{
    internal class Program
    {
        private static readonly Random RandomGenerator = new Random();
        private List<int> ArrayOf3xMatrix = new List<int>();


        static void Main(string[] args)
        {
            ArrayOf3xMatrix = FillMatrix(3);
        }

        private static List<int> FillMatrix(int size)
        {
            List<int> matrix = new List<int>();
            for (int i = 0; i < size * size * size; i++)
            {
                matrix.Add(GenerateRandomNumber());
            }
            return matrix;
        }
        private static void PrintRandomMatrix(int size)
        {
            for (int j = 0; j < size; j++)
            {
                for (int i = 0; i < size; i++)
                {
                    
                }
                Console.WriteLine();
            }
        }
        private static int GenerateRandomNumber()
        {
            return RandomGenerator.Next(0, 100);
        }

    }
}