You are not adding the newly created rebels or citizens into your map Dictionary, but just some string-int or string-string pairs, creating these objects is thus superfluous. Would be better to add string-object pair into your Dictionary and then infer the type of the each object with the help of the GetType and typeof operators (which you will definitely need for the exam).

Demo Code with List:

public class Engine3
    {
        public Engine3()
        {
        }

        public void Run()
        {
            int n = int.Parse(Console.ReadLine());
            List<Citizen> citizenList = new List<Citizen>();
            List<Rebel> rebelList = new List<Rebel>();

            AddMembersToLists(n, citizenList, rebelList);

            MembersBuyFood(citizenList, rebelList);

            int totalFood = 0;

            totalFood = TotalFoodCalculator(citizenList, rebelList, totalFood);

            Console.WriteLine(totalFood);
        }

        private static int TotalFoodCalculator(List<Citizen> citizenList, List<Rebel> rebelList, int totalFood)
        {
            foreach (var citizen in citizenList)
            {
                totalFood += citizen.Food;
            }

            foreach (var rebel in rebelList)
            {
                totalFood += rebel.Food;
            }

            return totalFood;
        }

        private static void MembersBuyFood(List<Citizen> citizenList, List<Rebel> rebelList)
        {
            string targetName = Console.ReadLine();

            while (targetName != "End")
            {
                if (citizenList.Any(cit => cit.Name == targetName))
                {
                    citizenList.First(cit => cit.Name == targetName).BuyFood();
                }
                else if (rebelList.Any(reb => reb.Name == targetName))
                {
                    rebelList.First(reb => reb.Name == targetName).BuyFood();
                }

                targetName = Console.ReadLine();
            }
        }

        private static void AddMembersToLists(int n, List<Citizen> citizenList, List<Rebel> rebelList)
        {
            for (int i = 0; i < n; i++)
            {
                string[] personInfo = Console.ReadLine()
                    .Split(" ", StringSplitOptions.RemoveEmptyEntries);

                if (personInfo.Length == 4)
                {
                    string name = personInfo[0];
                    int age = int.Parse(personInfo[1]);
                    string id = personInfo[2];
                    string birthdate = personInfo[3];
                    Citizen currentCitizen = new Citizen(name, age, id, birthdate);
                    citizenList.Add(currentCitizen);
                }
                else if (personInfo.Length == 3)
                {
                    string name = personInfo[0];
                    int age = int.Parse(personInfo[1]);
                    string group = personInfo[2];
                    Rebel currentRebel = new Rebel(name, age, group);
                    rebelList.Add(currentRebel);
                }
            }
        }
    }