How do I fix this.( im a beginner)

That isn't all your code: that code on it's own generates no errors, and runs, doing what you expected it to:

using System;
using System.Collections.Generic;
using System.IO;

public class Program
    {
    public static void Main()
        {
        int ans;
        Console.Write("2+3+1= ");
        ans=Convert.ToInt32(Console.ReadLine());
        if(ans==6) 
            {
            Console.WriteLine("Correct");
            }
        else
            {
            Console.WriteLine("wrong");
            }
        }
    }

Somewhere else in the same scope in your code, you are declaring something called "ans" - a variable, constant, or even a method!

And don't use Convert methods on user input - they will cause you app to crash if the user enters a non-numeric character. Use the TryParse methods instead: Int32.TryParse Method (System) | Microsoft Docs[^]. They return a bool "OK / Failed" result so you can give them another chane.