VBA reference issue - excel

Question

Im at a loss here, why do i get run-time error 91: "object variable or with block variable not set"
What am i doing wrong here and what's the fix?
Thank you in advance!
Set StartCell = Worksheets("Data").Range("A5")
Set StartSheet = Worksheets("Data")
With Worksheets("Data").Range("A4:BZ4")
Set LastColumn = .Find("Comment", LookIn:=xlValues)
Debug.Print StartCell.Row
Debug.Print StartCell.Column
Set Workrange = Range(StartSheet.Cells(StartCell.Row, StartCell.Column), StartSheet.Cells(5000, LastColumn.Column)) 'This line is the issue
End With
If Not Intersect(Target, Workrange) Is Nothing Then
If Target.Count > 1 Then Exit Sub
StartSheet.Cells(Target.Row, LastColumn.Column + 1).Value = Environ("username")
StartSheet.Cells(Target.Row, LastColumn.Column + 2).Value = Format(Now, "dd/mm/yyyy_hh.mm.ss")
End If
End Sub

Answer 1

Issues in your code
Find might not return a hit, test for that
Find uses the last set values for several parameters. Not specifying them can give unexpected results.
Other improvements
If you are going to exit on Target.Count > 1 then just do it
In a Worrksheet code behind module, use Me to refer to that sheet
Dim your variables
Removed redundant With
Cleaned up cumbersome range reference
Private Sub Worksheet_Change(ByVal Target As Range)
Dim StartCell As Range
Dim LastColumn As Range
Dim Workrange As Range
If Target.Count > 1 Then Exit Sub
Set StartCell = Me.Range("A5")
'Debug.Print StartCell.Row
'Debug.Print StartCell.Column
Set LastColumn = Me.Range("A4:BZ4").Find( _
What:="Comment", _
LookIn:=xlValues, _
LookAt:=xlWhole, _
SearchOrder:=xlByRows, _
MatchCase:=False, _
MatchByte:=TRUE) ' only need MatchByte if you have selected or installed double-byte language support.
If Not LastColumn Is Nothing Then
Set Workrange = Me.Range(StartCell, Me.Cells(5000, LastColumn.Column))
If Not Intersect(Target, Workrange) Is Nothing Then
Me.Cells(Target.Row, LastColumn.Column + 1).Value = Environ("username")
Me.Cells(Target.Row, LastColumn.Column + 2).Value = Format$(Now, "dd/mm/yyyy_hh.mm.ss")
End If
End If
End Sub