I am unaware on how to fix this conversion error

using System;
using System.Globalization;
using System.IO;


namespace LogIn
{
    public class Administrator : RegularUser
    {
        //data fields
        private int ID;


        public Administrator(int ID = 0, string userName = " ") : base(userName)
        {
            //data fields
            this.ID = ID;
        }

        //GETTER
        /**
         Returns the ID of the admin
        @return: admin's ID
         */
        public int GetID() { return ID; }


        //SETTERS
        /**
         Returns an updated ID number
        @return: admin's updated ID number
         */
        public void SetID(int ID) { this.ID = ID; }

        //sign-in method
        public new static void SignIn(string dateAndTime, StreamWriter writer)
        {
            writer.WriteLine("signed in @ {1}", dateAndTime);
        }

        //sign out method
        public new static void SignOut(string dateAndTime, StreamWriter writer)
        {
            writer.WriteLine("signed out @ {1}", dateAndTime);
        }


        /** Returns the user's username and ID in string format and whether they've signed in/out
          @return: a string representing who signed in/out along with ther ID #
      */
        public override string Stringify()
        {
            return string.Format("({1}) ", base.Stringify(), GetID());
        }

        public override string Stringify2()
        {
            return string.Format("({1}) ", base.Stringify2(), GetID());
        }


    }


}