Runtime error 1004 application defined or object defined error

I have a small macro that is manipulating columns and entering formula's in a couple of cells but for some reason when I try to set the vlookup formula I get a runtime error. (at this line  ActiveCell.FormulaR1C1 = Formula)

If I paste the string that is in Formula into the cell manually the formula is fine and works perfectly. Can someone please tell me why I can't paste it in using vba code?


Sub Macro1()
   
    Sheets("Sheet2").Select
    Dim Check As String
    Check = "D1:D" + CStr(LastRow(Sheets("Sheet2")))
    Columns("A:A").Select
    Selection.Cut
    Columns("F:F").Select
    Selection.Insert Shift:=xlToRight
    Columns("A:B").Select
    Selection.NumberFormat = "@"
    Range("D1").Select
    ActiveCell.FormulaR1C1 = "=CONCATENATE(RC[-3],RC[-2])"
    Selection.AutoFill Destination:=Range(Check)
    Sheets("Sheet1").Select
    Range("A1").Select
    ActiveCell.EntireColumn.Insert
    Dim DestRange As String
    DestRange = "A3:" + CStr(LastRow(Sheets("Sheet1")))
    Dim Formula As String
    Dim Rng As String
    Rng = "$D$1:$E$" + CStr(LastRow(Sheets("Sheet2")))
    Formula = "=VLOOKUP(CONCATENATE(C2,D2),Sheet2!" + Rng + ",2,FALSE)"
    Range("A2").Select
    ActiveCell.FormulaR1C1 = Formula
    Selection.AutoFill Destination:=Range(DestRange)
    Range("A1").Select
    ActiveCell.Value = "NEW_UCID"
    Range("B1").Select
    ActiveCell.Value = "OLD_UCID"
   
End Sub

Function LastRow(ws As Object) As Long

        Dim rLastCell As Object
        On Error GoTo ErrHan
        Set rLastCell = ws.Cells.Find("*", ws.Cells(1, 1), , , xlByRows, _
                                      xlPrevious)
        LastRow = rLastCell.Row

ErrExit:
        Exit Function

ErrHan:
        MsgBox "Error " & Err.Number & ": " & Err.Description, _
               vbExclamation, "LastRow()"
        Resume ErrExit

    End Function