Getting a "Run-time Error 91"

[figgylynn1023]

I am trying to create a macro that will help compare today's data sets with the data sets from the previous day. I want the macro to:
1. Open the previous day workbook, clear fields and sort column F in ascending order on the worksheet "Report Comparison" in that workbook.
2. Go back to the current day workbook, clear fields and sort column F in ascending order on the worksheet "Report Comparison" in that workbook.
3. Run a duplicates formula that will spit back "PDD" for previous day duplicates and "SDD" for Same Day Duplicates on the Current day worksheet (to be posted in column C on the "Report Comparison" worksheet of the current day wookbook).

However, with the code I have below, when I try to run the macro, it stops and give me the message "Run-time error '91': Object variable or With block variable not set'. (Where I've made the text red). I'm confused because I copied this code from another report that it worked for. Thoughts?

Sub Duplicates()
'
' Duplicates Macro
'
    Dim Day As String
    Dim Prev As String
Day = InputBox("Today's File(mmddyy):")
Prev = InputBox("Previous Date(mmddyy):")
' Opening Previous Day
    Workbooks.Open Filename:= _
        "\\Drive\Folder\Reports\Report_Comparison_" & Prev & "_All.xlsm"
    Windows("Report_Comparison_" & Prev & "_All.xlsm").Activate
        Selection.AutoFilter
    ActiveWorkbook.Worksheets("Report Comparison").AutoFilter.Sort.SortFields.Clear
    ActiveWorkbook.Worksheets("Report Comparison").AutoFilter.Sort.SortFields.Add _
        Key:=Range("F1:F65536"), SortOn:=xlSortOnValues, Order:=xlAscending, _
        DataOption:=xlSortTextAsNumbers
    With ActiveWorkbook.Worksheets("Report Comparison").AutoFilter.Sort
        .Header = xlYes
        .MatchCase = False
        .Orientation = xlTopToBottom
        .SortMethod = xlPinYin
        .Apply
    End With
' Back to Current Day
    Windows("Report_Comparison_" & Day & "_All.xlsm").Activate
        Selection.AutoFilter
    ActiveWorkbook.Worksheets("Report Comparison").AutoFilter.Sort.SortFields.Clear
    ActiveWorkbook.Worksheets("Report Comparison").AutoFilter.Sort.SortFields.Add _
        Key:=Range("F1:F65536"), SortOn:=xlSortOnValues, Order:=xlAscending, _
        DataOption:=xlSortTextAsNumbers
    With ActiveWorkbook.Worksheets("Report  Comparison").AutoFilter.Sort
        .Header = xlYes
        .MatchCase = False
        .Orientation = xlTopToBottom
        .SortMethod = xlPinYin
        .Apply
    End With
' Duplicates Formula
    Columns("A:C").Select
        Selection.NumberFormat = "General"
    Range("C2").FormulaR1C1 = _
        "=IFERROR(IF(VLOOKUP(RC[3],'[Report_Comparison_" & Prev & "_All.xlsm]Report Comparison'!C6,1,FALSE)=RC[3],""PDD"",""""),IF(RC[3]=R[1]C[3],""SDD"",""""))"
    Range("D2").Select
        Range(Selection, Selection.End(xlDown)).Select
        Selection.Copy
        Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks:=False, Transpose:=False
End Sub

 

[figgylynn1023]

I'm really hoping someone can help me with this because I'm kind of stuck until this gets figured out.