Macro to set Flag on dup values

[carlysyme]

I wrote this macro:

Private Sub Worksheet_Change(ByVal Target As Range)
Dim CurrentWorksheet As Worksheet
    
    Application.ScreenUpdating = False
    On Error GoTo ErrorHandler
    Set CurrentWorksheet = ThisWorkbook.ActiveSheet
    Dim MaxCount As Integer
    Dim StartCount As Integer
    Dim CountRange As String
    Dim LChangedValue As String

Application.ScreenUpdating = False
    'Test first 200 rows in spreadsheet for uniqueness
    MaxCount = 20
    StartCount = 6

    'Clear all flags
    CountRange = "A6:A" & MaxCount
    Range(CountRange).Interior.ColorIndex = xlNone

   
    While StartCount < MaxCount
        LChangedValue = "A" & CStr(StartCount)
        test = Application.WorksheetFunction.CountIf(Range("A6:A999"), "A" & CStr(StartCount))
       If test > 1 Then
          Range(LChangedValue).Interior.ColorIndex = 3
          Worksheets("Room and Bed").Range("B" & CStr(StartCount)).Value = "This is text"
       End If
       StartCount = StartCount + 1
    Wend

ErrorHandler:
    Application.EnableEvents = True
    Application.ScreenUpdating = True


End Sub

But it dosn't seem to work the worksheet function isn't working. Can anyone help is there something else I need to do or may have done.