Excel.Workbook not defined

To all

I am trying to write a macro to open an excel spreasheet from NX but I ma getting the error
Excel.Workbook not defined

could anyone help/make a suggesting on this one? Code is below

Thanks

JXB

'--------------------------------------
Option Strict Off
Imports System
Imports System.Threading
Imports NXOpen
Imports NXOpenUI
Imports MiniSnap, MiniSnap.Create

Module testxls

Public Sub Main
Dim FileTest As String = "H:\NXMacros\ExcelTestFile.xlsx"
Dim ExcelO As Object
Dim BookO As Excel.Workbook
Dim SheetO As Excel.Worksheet
ExcelO = CreateObject("Excel.Application")
BookO = ExcelO.activeWorkbook
SheetO = ExcelO.Worksheets(1)

Dim testvar1 As String = SheetO.Range("A1").Value
Dim testvar2 As String = SheetO.Range("A2").Value

Dim testvar3 As String = "This is my string to write"
SheetO.Range("A3").Value = testvar3

ExcelO.displayAlerts = False
BookO.SaveAs(FileTest, 51) 'Code 51 is xlsx
BookO.Close()
BookO = Nothing

End Sub
End Module