What is the error message meant when I call find.One using DirectorySearcher

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C#

Hi,

I'm new in writing code to access AD. I setup following AD structue:
DC=UAT,DC=COM
OU=ORG
OU=Users and Groups
OU=System
OU=Groups
CN=Admins
OU=Users
OU=Test
OU=Groups
OU=Users
CN=test01
CN=test02

I wrote the following sample codes to search for the user test02, but got error:

using System;
using System.IO;
using System.DirectoryServices;
using System.Configuration;
try
{
    DirectoryEntry userEntry = new DirectoryEntry("LDAP://SERVERIP/OU=ORG,DC=UAT,DC=COM", user, password); 
    DirectorySearcher search = new DirectorySearcher(userEntry);
             
    search.SearchScope = SearchScope.Subtree

    // Search user test02           
    search.Filter = "(&(objectClass=user)(objectCategory=Person)(sAMAccountName=test02))";               
             
    SearchResult result = search.FindOne();   // Failed at this line,  throw exception 

    if (result == null)
       Console.WriteLine("There is no results to display");
    else
       Console.WriteLine("Found result ");                
}
catch (Exception ex)
{
    Console.WriteLine("Error: " + ex.Message);
    Console.WriteLine("Trece: " + ex.StackTrace);
}

Error:  Configuration system failed to initialize<br />
Trece:    at  System.Configuration.ClientConfigurationSystem.EnsureInit(String configKey)<br />
   at System.Configuration.ClientConfigurationSystem.PrepareClientConfigSystem(String sectionName)<br />
   at System.Configuration.ClientConfigurationSystem.System.Configuration.Internal.IInternalConfigSystem.GetSection(String sectionName)<br />
   at System.Configuration.ConfigurationManager.GetSection(String sectionName)<br />
   at System.Configuration.PrivilegedConfigurationManager.GetSection(String sectionName)<br />
   at System.DirectoryServices.SearchResultCollection.ResultsEnumerator..ctor(SearchResultCollection results, String parentUserName, String parentPassword, AuthenticationTypes parentAuthenticationType)<br />
   at System.DirectoryServices.SearchResultCollection.GetEnumerator()<br />
   at System.DirectoryServices.DirectorySearcher.FindOne()<br />

Could you help and advise what I did wrong?

Thank you very much!
Lwang