Code wont work in new workbook

I have some code that works well for a Table of contents on the Workbook Index page, that chooses the heading of each sheet (C5 of each sheet), and puts the headings in a "table of contents list" on the index page (starting in C11) (see code below). It allows a table of contents to "list/show" only "un-hidden sheets".
The problem I am having is I'm trying to put it into a new Workbook (same types of pages) but I'm not sure how to "change / edit" it to work.
My new workbook is named "NEWEST NEW" (If it matters).
BTY, I'm very green at this. Any Help?
Thanks / Richard


[Option Explicit

'=========================================================================================
'=========================================================================================


Sub UpdateIndexWorksheet()

Const sINDEX_SHEETNAME As String = "Workbook Index"
Const sSHEETNAME_CELL As String = "C5"
Const sHEADER_CELL As String = "C10"

Dim rHeaderCell As Range
Dim wksIndex As Worksheet
Dim iRowNo As Integer
Dim wks As Worksheet

Set wksIndex = ThisWorkbook.Worksheets(sINDEX_SHEETNAME)

Set rHeaderCell = wksIndex.Range(sHEADER_CELL)

Call ClearIndexCells(wksIndex:=wksIndex, rHeaderCell:=rHeaderCell)

iRowNo = 1

For Each wks In ThisWorkbook.Worksheets

If wks.Visible = xlSheetVisible Then
rHeaderCell.Offset(iRowNo, 0).Value = wks.Range(sSHEETNAME_CELL)
iRowNo = iRowNo + 1
End If

Next wks

End Sub

'=========================================================================================
'=========================================================================================

Private Sub ClearIndexCells(wksIndex As Worksheet, rHeaderCell As Range)

Dim rFirstCell As Range
Dim rLastCell As Range
Dim rLastRow As Range

Set rFirstCell = rHeaderCell.Offset(1, 0)

With wksIndex
Set rLastRow = .UsedRange.Rows(.UsedRange.Rows.Count)
End With

Set rLastCell = Intersect(rLastRow, rHeaderCell.EntireColumn)

Range(rFirstCell, rLastCell).ClearContents

End Sub]

Why not "save as" the old workbook with new workbook name.