Runtime error usually only occurs when some part of the code is not working, so it could be that the current amount of eggs should only be taken from the console when the commands "Buy" and "Fill" are received. I've also included the break validation inside of the "Buy" command. Judge is not running at the moment until Monday, so try to see then if the corrections are valid.

Best,

Code:

using System;

namespace _04._Easter_Shop
{
    class Program
    {
        static void Main(string[] args)
        {
            int currentQuantityEgss = int.Parse(Console.ReadLine());
            int buyedEgss = 0;

            while (true)
            {
                string command = Console.ReadLine();

                if (command == "Close")
                {
                    Console.WriteLine("Store is closed!");
                    Console.WriteLine($"{buyedEgss} eggs sold.");
                    break;
                }

                //int eggsNumber = int.Parse(Console.ReadLine());

                if (command == "Buy")
                {
                    int eggsNumber = int.Parse(Console.ReadLine());
                    if (eggsNumber > currentQuantityEgss)
                    {
                        Console.WriteLine("Not enough eggs in store!");
                        Console.WriteLine($"You can buy only {currentQuantityEgss}.");
                        break;
                    }
                    currentQuantityEgss -= eggsNumber;
                    buyedEgss += eggsNumber;
                }
                else if (command == "Fill")
                {
                    int eggsNumber = int.Parse(Console.ReadLine());
                    currentQuantityEgss += eggsNumber;
                }

                //if (eggsNumber > currentQuantityEgss && command == "Buy")
                //{
                //    Console.WriteLine("Not enough eggs in store!");
                //    Console.WriteLine($"You can buy only {currentQuantityEgss}.");
                //}
            }
        }
    }
}