Easiest way is to use some bool validation.

Best,

using System;

namespace MyFirstProject2
{
    class Program
    {
        static void Main(string[] args)
        {
            string fruit = Console.ReadLine();

            string day = Console.ReadLine();

            bool validPrice = false;

            double num = double.Parse(Console.ReadLine());

            double price = 0;
            if (day == "Monday" || day == "Tuesday" || day == "Wednesday" || day == "Thursday" || day == "Friday")
            {
                if (fruit == "banana")
                {
                    price = num * 2.50;
                    validPrice = true;
                }
                else if (fruit == "apple")
                {
                    price = num * 1.20;
                    validPrice = true;
                }
                else if (fruit == "orange")
                {
                    price = num * 0.85;
                    validPrice = true;
                }
                else if (fruit == "grapefrui")// ERROR grapefruit!!!
                {
                    price = num * 1.45;
                    validPrice = true;
                }
                else if (fruit == "kiwi")
                {
                    price = num * 2.70;
                    validPrice = true;
                }
                else if (fruit == "pineapple")
                {
                    price = num * 5.50;
                    validPrice = true;
                }
                else if (fruit == "grapes")
                {
                    price = num * 3.85;
                    validPrice = true;
                }
            }
            else if (day == "Saturday" || day == "Sunday")
            {
                if (fruit == "banana")
                {
                    price = num * 2.70;
                    validPrice = true;
                }
                else if (fruit == "apple")
                {
                    price = num * 1.25;
                    validPrice = true;
                }
                else if (fruit == "orange")
                {
                    price = num * 0.90;
                    validPrice = true;
                }
                else if (fruit == "grapefrui") // ERROR grapefruit !!!
                {
                    price = num * 1.60;
                    validPrice = true;
                }
                else if (fruit == "kiwi")
                {
                    price = num * 3.00;
                    validPrice = true;
                }
                else if (fruit == "pineapple")
                {
                    price = num * 5.60;
                    validPrice = true;
                }
                else if (fruit == "grapes")
                {
                    price = num * 4.20;
                    validPrice = true;
                }
            }

            if (validPrice == false)
            {
                Console.WriteLine("error");
            }
            else if (validPrice == true)
            {
                Console.WriteLine($"{price:f2}");
            }
        }
    }
}