ConfigurationManager is Not Declared????

I have been trying to get my connectionstring from the web.config file and both methods I have tried give errors.
I have imported the System.Configuration but I keep getting ConfigurationManager not declared.
I also tried using ConfigurationSettings.appsettings and get this is obsolete.
What am I doing wrong???
Thanks

Imports System
Imports System.Collections.Generic
Imports System.Text
Imports System.Data
Imports System.Configuration
Imports System.Data.Common
Imports System.Data.SqlClient
Imports System.Data.OleDb
Imports System.Data.Odbc
Imports System.IO
 
Namespace ApplicationFramework.DataAccessLayer
    Public NotInheritable Class DBManager
        Inherits DBManagerBase
        Public Sub OpenConnection()
 
            ConnectionString = ConfigurationManager.ConnectionStrings("ConnectionString").ConnectionString
          
          '  ConnectionString = ConfigurationSettings.AppSettings("ConnectionString").ToString
            MyBase.Open(ConnectionString)
        End Sub

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