Try this alternative
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Const WS_RANGE As String = "D:E" '<=== change to suit
On Error GoTo err_handler
Application.EnableEvents = False
If Not Application.Intersect(Target, Range(WS_RANGE)) Is Nothing Then
.Font.Name = "Marlett"
Select Case .Value
Case "": .Value = "a"
Case "a": .Value = ""
Application.EnableEvents = True
This is worksheet event code, which means that it needs to be
placed in the appropriate worksheet code module, not a standard
code module. To do this, right-click on the sheet tab, select
the View Code option from the menu, and paste the code in.
(there's no email, no snail mail, but somewhere should be gmail in my addy)