Last row issues - excel

Question

I'm having some issues with last row expression and autofill. I'm not sure what I'm missing here as it looks correct but it doesn't seem to be starting the vlookup in the proper cell (N2, it starts it in N1) and it won't autofill to the last row of M. Any push in the right direction would be greatly appreciated. I'm thinking it's something small I'm overlooking.
Sub Nightly()
'
' Nightly Macro
'
Dim PackSpec As Workbook
Dim FullBook As Workbook
Dim DebFile As Workbook
Dim lrow As Long
lrow = Cells(Rows.Count, 1).End(xlUp).Row
'Open the nightly pack spec file, cut and insert the year row into column D
Set PackSpec = Workbooks.Open("S:\Accounting\Apps\Packspec\CIDExport\Archive\" & Year(Date) & "\" & Month(Date) & "\" & Day(Date) - 1 & "\*.csv")
Columns("A:A").Select
Selection.Cut
Columns("D:D").Select
Selection.Insert Shift:=xlToRight
Range("A2").Select
'Open Fullbook master and insert columns after N then VLookup between Pack Spec and Fullbook
Set FullBook = Workbooks.Open("S:\Corporate\Groups\Comosoft\Downloads\FullBook\fullbook_Master.csv")
Columns("N:U").Select
Selection.Insert Shift:=xlToRight
Range("N2").Select
'Actvate Fullbook and enter Vlookup for dates
Windows("fullbook_Master.csv").Activate
With ActiveSheet.Range("N2")
.FormulaR1C1 = "=VLOOKUP(RC[-1],'[15.50.1.CID.csv]15.50.1.CID'!C[-13]:C[-11],3,0)"
.AutoFill Destination:=Range("N2:N" & lrow) 'issue not autofilling to end
Windows("fullBook_Master.csv").Activate
End With
End Sub

Answer 1

Just move the
lrow = Cells(Rows.Count, 1).End(xlUp).Row
after
Windows("fullbook_Master.csv").Activate

Answer 2

You should:
Link lrow to a sheet to avoid mistakes.
Avoid all the selects.
Sub Nightly()
'
' Nightly Macro
'
Dim PackSpec As Workbook
Dim FullBook As Workbook
Dim DebFile As Workbook
Dim lrow As Long
Dim ws As Worksheet, wsPackSpec As Worksheet
Set ws = Activesheet
'Or set ws = Sheets("Sheet1") - better
'Open the nightly pack spec file, cut and insert the year row into column D
Set PackSpec = Workbooks.Open("S:\Accounting\Apps\Packspec\CIDExport\Archive\" &
Year(Date) & "\" & Month(Date) & "\" & Day(Date) - 1 & "\*.csv")
Set wsPackSpec = PackSpec.Sheets(1)
wsPackSpec.Columns(1).Value = wsPackSpec.Columns(4).Value
wsPackSpec.Columns(1).EntireColumn.Delete
'Open Fullbook master and insert columns after N then VLookup between Pack Spec and Fullbook
'Apply same principle as above here
Set FullBook = Workbooks.Open("S:\Corporate\Groups\Comosoft\Downloads\FullBook\fullbook_Master.csv")
Columns("N:U").Select
Selection.Insert Shift:=xlToRight
Range("N2").Select
'Actvate Fullbook and enter Vlookup for dates
Windows("fullbook_Master.csv").Activate
Set ws = ActiveSheet 'Set like this, but you should designate the correct worksheet in the fullbook part above
lrow = ws.Cells(ws.Rows.Count, 1).End(xlUp).Row
With ws
.Cells(2, 14).FormulaR1C1 = "=VLOOKUP(RC[-1],'[15.50.1.CID.csv]15.50.1.CID'!C[-13]:C[-11],3,0)"
.Cells(2, 14).AutoFill Destination:=.Range(.Cells(2, 14), .Cells(lrow, 14))
End With
End Sub
Something like this, couldn't test in detail because I'm missing the overview of how the books are set up.