Can someone solve and fix this error for me

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Hacker : MonoBehaviour {

    int level;

    enum Screen {MainMenu, Password, Win};
    Screen currentScreen = Screen.MainMenu;

    // Start is called before the first frame update
    void Start()
    {

        showMainMenu();

    }

    void showMainMenu() {
        Terminal.ClearScreen();
        Terminal.WriteLine("Welcome to the Terminal Hacker by Meet");
        Terminal.WriteLine("");
        Terminal.WriteLine("Press 1 to Hack Into the Local Library");
        Terminal.WriteLine("Press 2 to Hack Into the Server");
        Terminal.WriteLine("Press 3 to Hack Into NASA");
        Terminal.WriteLine("");
        Terminal.WriteLine("Enter your Selection: ");

    }

    void OnUserInput(string input) {

        if (input == "menu") {

            showMainMenu();

        } 

         else if(currentScreen == Screen.MainMenu)
        {
            RunMainMenu(input);

            Terminal.WriteLine("Please choose a valid option");

        }

        void StartGame() {

                Terminal.WriteLine("You have chosen level " + level);

        }

        void RunMainMenu(string input)
        {
            if (input == "1")
            {

                level = 1;
                StartGame();

            }
            else if (input == "2")
            {

                level = 2;
                StartGame();


            }
            else if (input == "3")
            {

                level = 3;
                StartGame();

            }
        }
    }

    // Update is called once per frame
    void Update()
    {
        
    }


}

Error which I am getting in unity:

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