Adding a note to a cell based on another cell - excel

Question

I am trying to add a specific note to a cell based on the cell value to explain what the cell contents are. I am trying to use the code below to do this but I get a run-time error '1004' on the following line:
Target.Cells.Comment.Text Text:=Comment_E
Private Sub Worksheet_Change(ByVal Target As Range)
'Defining what column is being changed
If Target.Column = 3 Then
'Adding a comment to the cell
Dim Status_Col As String
Dim NA As String
Dim i As Integer
'Types
Dim Comment_T As String
'Explainations
Dim Comment_E As String
i = 4
Comment_T = Target.Cells.Value
For i = 4 To 10 ' checking the list of types
If Cells(i, 14).Value = Comment_T Then
Comment_E = Cells(i, 15).Value
End If
Next i
Target.Cells.Select
Target.Cells.AddComment
Target.Cells.Comment.Visible = False
Target.Cells.Comment.Text Text:=Comment_E
Selection.ShapeRange.ScaleHeight 0.48, msoFalse, msoScaleFromTopLeft
Selection.ShapeRange.ScaleWidth 3.32, msoFalse, msoScaleFromTopLeft
End If
End Sub

Answer 1

You need to account for the case where a cell already has a comment, and the case where multiple cells are updated (eg fill down).
Few fixes/suggestions:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim v, cmt As Comment, rng As Range, c As Range
'any changes in Col3?
Set rng = Application.Intersect(Target, Me.Columns(3))
'optionally set some limit for the size of change you want to handle
If rng.Cells.CountLarge > 100 Then Exit Sub
'now process each cell in the col3 range
For Each c In rng.Cells
v = Application.VLookup(c.Value, Me.Range("N4:O10"), 2, False)
If Not IsError(v) Then
Set cmt = c.Comment 'already has a comment?
If cmt Is Nothing Then
Set cmt = c.AddComment() 'no comment so add one
With cmt 'formatting...
.Visible = False
.Shape.Height = 30 'fixed height/width is easier
.Shape.Width = 100
End With
End If
cmt.Text v 'set/replace text
End If
Next c
End Sub