The problem in your solution was that you already applied Math.Floor on: var liter = Math.Ceiling(grape / 2.5), before you entered the if-else validation. This most likely led to some calculation errors by the judge system :

Refactored code (100%):

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace _04.Harvest
{
    class Program
    {
        static void Main(string[] args)
        {
            var X = double.Parse(Console.ReadLine()); // площта на лозето
            var Y = double.Parse(Console.ReadLine()); //от 1 кв.м. се изкарват У кг грозде 
            var targetLiter = double.Parse(Console.ReadLine()); //нужни литри вино 
            var workers = int.Parse(Console.ReadLine()); // брой работници

            //var total_grape = X * 0.4;
            //var grape = Y * total_grape;
            //var actualLiter = grape / 2.5;

            var amountGrapes = ((X * Y) / 2.5);
            var actualLiter = amountGrapes * 0.4;

            if (actualLiter < targetLiter)
            {
                var needed = Math.Floor(targetLiter - actualLiter);
                Console.WriteLine("It will be a tough winter! More {0} liters wine needed.", needed);
            }
            else
            {
                var leftOver = Math.Ceiling(actualLiter - targetLiter);
                var person = Math.Ceiling(leftOver / workers);
                Console.WriteLine("Good harvest this year! Total wine: {0} liters.", Math.Floor(actualLiter));
                Console.WriteLine("{0} liters left -> {1} liters per person. ", leftOver, person);
            }
        }
    }
}