Event Handler -- Cell not Updating as Expected - excel

Question

The code:
Private Sub Worksheet_change(ByVal target As Range)
Application.ScreenUpdating = False
Application.EnableEvents = False
Dim cell
For Each cell In Me.UsedRange.Columns("E").Cells
If cell.Text = "Cu" And cell.offset(0, -1) = "WR229" Then
MsgBox "Cu not permitted for WR229 or larger waveguide", vbOKOnly, "Cu Alert"
cell = "Al"
End If
Next cell
Application.ScreenUpdating = True
Application.EnableEvents = True
End Sub
The problem is that when the condition is met the cell does not get reset to the value "Al". Why not?

Answer 1

One possible reason the OP's code can fail is if the UsedRange does not start in column A. This will occur if there is no data and no formatting at all in column A.
Why? Because .Columns (and .Rows and .Cells for that matter) are relative to the specified range. For example if the UsedRange is B2:Z10 then Me.UsedRange.Columns("E") will refer to range F2:F10.
Another issue in the OP's code is that it will run for any cells change, including those in Column A. This will throw an error because Offset -1 from Column A is invalid.
So, how to fix it? As answered by jbarker2160, you should take advantage of the Target parameter, which tells you which cells have changed. However that answer leaves several problems.
We want to check for Column E = "Cu" and Column D for "WR229", but we don't know which one will be entered first
We should account for the possibility that several cells are changed at once, eg due to a copy/paste
we should handle possible errors and not leave Events disabled
The OP's code is case sensitive. Ie "CU" will be accepted when "Cu" is not. Is this the desired behaviour? (If it is, remove the UCase$()'s from the code below)
Implicit in the OP's code is that there is a range of larger waveguides that are disallowed for Cu. This remains un-addressed in the code below.
This code addresses the above issues
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rw As Range
On Error GoTo EH
Application.ScreenUpdating = False
Application.EnableEvents = False
For Each rw In Application.Intersect( _
Target.EntireRow, Me.UsedRange.EntireRow.Columns("D:E")).Rows
If UCase$(rw.Cells(1, 2)) = "CU" And UCase$(rw.Cells(1, 1)) = "WR229" Then
MsgBox "Cu not permitted for WR229 or larger waveguide", _
vbOKOnly, "Cu Alert"
rw.Cells(1, 2) = "Al"
End If
Next
EH:
Application.ScreenUpdating = True
Application.EnableEvents = True
End Sub

Answer 2

Code
Private Sub Worksheet_change(ByVal target As Range)
Application.ScreenUpdating = False
Application.EnableEvents = False
If target = "Cu" And target.offset(0, -1) = "WR229" Then
MsgBox "Cu not permitted for WR229 or larger waveguide", vbOKOnly, "Cu Alert"
target = "Al"
End If
Application.ScreenUpdating = True
Application.EnableEvents = True
End Sub
Explanation
Since you are doing this loop for each change, you don't need to loop through the entire column and using target will get around having to use the Value property.

Answer 3

Change this line:
cell = "Al"
To this:
cell.Value = "Al"