I removed the 'with' statements, cause that made it not work for me.
Code below works fine.

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column <> 8 Or Target.Row > 100 Then Exit Sub
If IsEmpty(Target) Then Exit Sub
Dim nt$, co$, cn$
nt = Target.Text
If Not Target.Comment Is Nothing Then
co = Target.Comment.Text & Chr(10) & Chr(10)
Else
co = ""
End If
On Error Resume Next
Target.Comment.Delete
Err.Clear
Target.AddComment
Target.Comment.Visible = False
Target.Comment.Text Text:=co & Format(VBA.Now, "MM/DD/YYYY at h:MM AM/PM") & Chr(10) & nt
Target.Comment.Shape.TextFrame.AutoSize = True
End Sub
Please click the * below if this helps