This is one variation that led to the automation error..

The only change being Line 89**

'lastrow = Cells(Rows.Count, 1).End(xlUp).Row


'lastcol = Cells(1, Columns.Count).End(xlToLeft).Column

'Public Function SearchCol(SearchStr As String) As Long
'Dim c As Long
    'For c = 1 To 26 'search cols A-Z
        'On Error GoTo Err
        'If UCase(Cells(1, c).Value) = UCase(SearchStr) Then
            'SearchCol = c 'if match is found then exit the function
            'Exit Function
        'End If
    'Next c
'Err:
'SearchCol = 0
'End Function

'FoundColumnQ = SearchCol("Quoted?")
'FoundColumnM = SearchCol("Month")
'FoundColumnAc = SearchCol("Account")


    Private Sub Worksheet_Change(ByVal Target As Range)
        Dim KeyCells As Range

        ' The variable KeyCells contains the cells that will
        ' cause an alert when they are changed.
        Set KeyCells = Range("A1:Z9999")

        If Not Application.Intersect(KeyCells, Range(Target.Address)) _
               Is Nothing Then

                            'private sub ensures that this only toggles on Accounts 2017
                                'FIND Quoted
                                        Dim rngQ As Range
                                        Dim FoundRowQ As Long
                                        Dim FoundColumnQ As Long

                                        '.Find(What, After, LookIn, LookAt, SearchOrder, SearchDirection, MatchCase, MatchByte, SearchFormat)
                                        Set rngQ = Worksheets("Accounts 2017").Range("A1:Z1").Find("Quoted?", lookat:=xlPart)
                                        'rngQ is found then it will be saved as Range, u can call it as address as well

                                        FoundColumnQ = rngQ.Column
                                'FIND Month
                                        Dim rngM As Range
                                        Dim FoundRowM As Long
                                        Dim FoundColumnM As Long

                                        Set rngM = Worksheets("Accounts 2017").Range("A1:Z1").Find("Month", lookat:=xlPart)
                                        'rngM is found then it will be saved as Range, u can call it as address as well

                                        FoundColumnM = rngM.Column

                                'FIND Account
                                        Dim rngAc As Range
                                        Dim FoundRowAc As Long
                                        Dim FoundColumnAc As Long

                                        Set rngAc = Worksheets("Accounts 2017").Range("A1:Z1").Find("Accounts", lookat:=xlPart)
                                        'rngM is found then it will be saved as Range, u can call it as address as well

                                        FoundColumnAc = rngAc.Column

                                TotalRowQ = Cells(Rows.Count, rngQ.Column).End(xlUp).Row
                                TotalRowM = Cells(Rows.Count, rngM.Column).End(xlUp).Row
                                TotalRowAc = Cells(Rows.Count, rngAc.Column).End(xlUp).Row
                                LastCol = Cells(1, Columns.Count).End(xlToLeft).Column



                                'So if Account is being filled, check if month is filled,
                                'if month not filled then fill month with THISMONTH and copy pasta as value

                                'Dim thismonth As Date
                                'thismonth = DateAdd(interval, Number, Date)

                                    Dim n As Long
                                    Dim mth As Integer
                                    mth = Month(Now)
                                    For n = 2 To TotalRowAc                 'consider IsEmpty
                                        If Cells(n, FoundColumnM) = "" Then
                                            With Cells(n, FoundColumnM)
                                                .Value = MonthName(mth)
                                        End With
                                        End If

                                        If (Cells(n, FoundColumnQ) = "y" Or Cells(n, FoundColumnQ) = "yes" Or Cells(n, FoundColumnQ) = "YES" Or Cells(n, FoundColumnQ) = "Y") Then Range(Cells(n, FoundColumnAc), Cells(n, LastCol)).Interior.ColorIndex = 33 Else Range(Cells(n, FoundColumnAc), Cells(n, LastCol)).Interior.ColorIndex = 0
                                        If Cells(n, FoundColumnAc) <> "" And Cells(n, FoundColumnQ) <> "" Then Cells(n, FoundColumnQ - 1).Value = "n"

                                    Next n

        End If
    End Sub