That was a difficult one to find. Basically your validation for the final output messages had to be modified and specifiy whether all people had been transported and any cabin still had some free places or whether all cabins were filled up with people. Without this specification, one of the tests would fail, most probably in the scenario where there were 0 people in the line and the code couldn't differentiate which final message to pick (with full or still empty cabins).

New code(100%)

using System;
using System.Linq;

namespace _2.TheLift
{
    class Program
    {
        static void Main(string[] args)
        {
            int peopleWaitingForLyft = int.Parse(Console.ReadLine());
            int[] wagon = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int peopleOnTheCurrentWagon = 0;
            int peopleonTheLyft = 0;
            bool NoMorePeople = false;

            for (int i = 0; i < wagon.Length; i++)
            {
                while (wagon[i] < 4)
                {
                    wagon[i]++;
                    peopleOnTheCurrentWagon++;
                    if (peopleonTheLyft + peopleOnTheCurrentWagon == peopleWaitingForLyft)
                    {
                        NoMorePeople = true;
                        break;
                    }
                }
                peopleonTheLyft += peopleOnTheCurrentWagon;
                if (NoMorePeople)
                {
                    break;
                }
                peopleOnTheCurrentWagon = 0;
            }

            if (peopleWaitingForLyft > peopleonTheLyft)
            {
                Console.WriteLine($"There isn't enough space! {peopleWaitingForLyft - peopleonTheLyft} people in a queue!");
                Console.WriteLine(string.Join(" ", wagon));
            }
            else if (peopleWaitingForLyft < wagon.Length * 4 && wagon.Any(w => w < 4))
            {
                Console.WriteLine("The lift has empty spots!");
                Console.WriteLine(string.Join(" ", wagon));
            }
            //else if (peopleonTheLyft == peopleWaitingForLyft && NoMorePeople == true)
            else if (wagon.All(w => w == 4) && NoMorePeople == true)
            {
                Console.WriteLine(string.Join(" ", wagon));
            }
        }
    }
}