Run Time Error 9 " Script out of range

Dear Friends

I write below formula, but no luck getting Run time error 9, attached workbook for further information, getting error on underlined area

Private Sub CommandButton1_Click()
  Dim rng As Range
  Dim temp As Variant
  Dim I As Long
  Dim a As Long
  Dim rng_dest As Range
  Dim LastRow As String
  Dim wkb As Workbook
  Dim wks As Worksheet
    
  Application.ScreenUpdating = False
'*******************************************************************************************************
  Set wkb = Workbooks.Open("D:\Purchase Order Data\Purchase Order Data.xlsb", Password:="vv2325", WriteResPassword:="vv2325")
  Set wks = wkb.Sheets("PO Data")
  LastRow = wks.Range("E1048576").End(xlUp).Row + 1
  
  I = 1
  
  Set rng_dest = wks.Range("E:O")
  
  ' Find first empty row in columns D:J on sheet Master Data
  
  Do Until WorksheetFunction.CountA(rng_dest.Rows(I)) = 0
  
    I = I + 1
    
  Loop
  
  'Copy range B16:I34 on sheet Purchase Order to Variant array
  
  Set rng = Sheets("Purchase Order").Range("B15:L34")
  

  
  For a = 1 To rng.Rows.Count
  
    If WorksheetFunction.CountA(rng.Rows(a)) <> 1 Then
    
      rng_dest.Rows(I).Value = rng.Rows(a).Value
      
      'Copy Purchase Order number
  
      wks.Range("A" & LastRow).Value = Sheets("Purchase Order").Range("I3").Value
        
      'Copy PO Number2
      
      'wks.Range("M" & LastRow).Value = Sheets("Purchase Order").Range("J3").Value
        
        
      'Copy Date
      
      wks.Range("B" & LastRow).Value = Sheets("Purchase Order").Range("I4").Value
      
      'Copy Company name
      
      wks.Range("C" & LastRow).Value = Sheets("Purchase Order").Range("C8").Value
      
       'Copy PO Value
      
      wks.Range("D" & LastRow).Value = Sheets("Purchase Order").Range("L41").Value
      
      
      I = I + 1
        ActiveWorkbook.RefreshAll
    End If
    
  Next a
          
  Application.ScreenUpdating = True
End Sub

Thanks in advance
Vaibhav