This should work, two mistakes corrected in the 'spend' and 'save' commands,

using System;
                    
public class Program
{
    public static void Main()
    {
        double neededMoney = double.Parse(Console.ReadLine());
        double ownedMoney =double.Parse(Console.ReadLine());
        int daysCounter= 0;
        int spendCounter = 0;

        while(ownedMoney < neededMoney && spendCounter < 5)
        {
            string command =Console.ReadLine();
            double money = double.Parse(Console.ReadLine());
            daysCounter++;
            if(command == "spend")
            {
                spendCounter++;
            
                if(ownedMoney - money < 0)
                {
                    ownedMoney = 0;
                }
                else
                {
                    ownedMoney -= money;
                }
            }
            else if(command == "save")
            {
                ownedMoney += money;  
                spendCounter = 0;  
            }
        }

        if(spendCounter==5)
        {
            Console.WriteLine("You can't save the money.");
            Console.WriteLine(daysCounter);
        }

        if(ownedMoney >= neededMoney)
        {
            Console.WriteLine($"You saved the money for {daysCounter} days.");
        }
    }
}