VBA to prompt when clicked on protected cell

[Manuprasad]

I have Workbook with both Sheet/workbook is protected. I have a code to lock/disable certain range of cells when the drop-down value "no" And unlock/enable when value of drop down is "yes" Whereas, drop-down value and cells I would like to disable are on different sheets.

Dropdown on "Main Sheet" Range of cells on "Sub Sheet"

I also need to throw a prompt to user when he clicks on protected range and when the value is set to "No".

I am using following code on "Main Sheet"

[/COLOR]Private Sub Worksheet_Change(ByVal Target As Range)
Dim worksh As Integer
Dim worksheetexists As Boolean
Dim str1 As String
with ThisWorkbook.Sheets("SubSheet")
  If UCase$(Range("E30").Value) = "YES" Then
    .Range("E20:I3019").Locked = False
  Else
    .Range("E20:I3019").Locked = True
  End If
end with
End If

Following code on "Subsheet"

Private Sub Worksheet_Change(ByVal Target As Range)
If Range("E20:I3019").Locked = True Then
If Intersect(Target, Range("$E$19:$I$3000")) Is Nothing Then Exit Sub
MsgBox "Please select the appropriate dropdown on ARF Sheet"
With Application
    .EnableEvents = False
    .Undo
    .EnableEvents = True
End With
End If
End Sub

But my concern is Its throwing prompt when cells are unlocked/enabled. I want it to throw a prompt when cells are locked/disabled. Please help