not all code paths return value in C# code

In that case you need to return the addition result from your Add Method.
Try this:

using System;

namespace Property_Get_Set_2
{

    class input
    {
        private int num1, num2, result;
        public int add()
        {

            result = num1 + num2;
            return result;
        }

        public int Number1
        {
            get
            {
                return num1;
            }

            set
            {
                num1 = value;
            }
        }

        public int Number2
        {
            get
            {
                return num2;
            }

            set
            {
                num2 = value;
            }
        }
    }

    class Program
    {

        static void Main(string[] args)
        {

            input Inpt = new input();

            Console.Write("Enter First Number : ");

            Inpt.Number1 = Convert.ToInt32(Console.ReadLine());

            Console.Write("Enter Second Number : ");

            Inpt.Number2 = Convert.ToInt32(Console.ReadLine());

            int resultFromMethod = Inpt.add();

            Console.WriteLine("Sum of Number is : \t" + resultFromMethod);

            Console.ReadLine();

        }

    }

}