doesn't open file despite of it's really existed

Hi  Guys  

I  no  know  why  shows  this error 

"Sorry We Could Not Find FILE1.xlsx. Is it possible it was moved, renamed or deleted"?

  in  this  line 

 Set Workbook1 = Workbooks.Open(File1)

despite  of  I'm  really  sure  this  file  is  existed  and  matched  with  name  and  I  check  it  by  another  macro  and  open  it  without  any problem  . I  no  know  why  show  this  error!! 

Sub Compare_Two_Files()

    Dim Workbook1 As Workbook
    Dim Workbook2 As Workbook
    Dim Rng1 As Range
    Dim Rng2 As Range
    Dim i As Integer, j As Integer

    File1 = "File1.xlsx"
    File2 = "File2.xlsx"
    File1_Sheet = "Sheet1"
    File2_Sheet = "Sheet1"

    Set Workbook1 = Workbooks.Open(File1)
    Set Workbook2 = Workbooks.Open(File2)

    Set Rng1 = Workbook1.Worksheets(File1_Sheet).UsedRange
    Set Rng2 = Workbook2.Worksheets(File2_Sheet).UsedRange
    Count = 1
    For i = 1 To Rng1.Rows.Count
        For j = 1 To Rng1.Columns.Count
            If Rng1.Cells(i, j) <> Rng2.Cells(i, j) Then
                For k = 1 To Rng1.Rows.Count
                    Sheet1.Cells(Count, k) = Rng2.Cells(i, k)
                Next k
                Count = Count + 1
                Exit For
            End If
        Next j
    Next i
    Workbook1.Close
    Workbook2.Close
    Set Rng1 = Nothing
    Set Rng2 = Nothing
    Set Workbook1 = Nothing
    Set Workbook2 = Nothing
End Sub

anybody  can  guid  me  what's  my  mistake ,please?