Display warning based on cell value

Hello,

I have a drop down list that has values "yes"/"no" (cell F 20) and button in the next cell ( cell G 20).
If user, selects "yes", button will be visible which he could click to add "Sheets"
If he selects "no", button will be hidden.

My question : I have write a code that will throw a warning if user has selected "yes" and not added new sheet and it should revert the value from drop down to "no" in this case.

I am not sure, what to include in my code that will serve my purpose?

Code on worksheet

Private Sub Worksheet_Change(ByVal Target As Range)
Dim worksh As Integer
Dim worksheetexists As Boolean
Dim str1 As String
ThisWorkbook.Unprotect Password:="xyz"
If Target.Address = "$F$20" Then
    Select Case UCase(Target)
        Case Is = "YES": Shapes("Button 8").Visible = msoTrue
        Case Is = "NO": Shapes("Button 8").Visible = msoFalse

Code on Module

Sub insertSheet()
    Application.ScreenUpdating = False
    Dim worksh As Integer
    Dim worksheetexists As Boolean
    Dim ws As Worksheet
    worksh = Application.Sheets.Count
    worksheetexists = False
    ThisWorkbook.Unprotect Password:="xyz"
    For x = 1 To worksh
        If Worksheets(x).Name = "Sheet" Then
            worksheetexists = True
            MsgBox "Sheet Already Exists"
            'Debug.Print worksheetexists
            Exit For
        End If
    Next x
    If worksheetexists = False Then
       Sheets("BrownSheet").Visible = True
        ActiveWorkbook.Sheets("BrownSheet").Copy _
       After:=ActiveWorkbook.Sheets("BrownSheet")
       Sheets("BrownSheet").Visible = False
       ActiveSheet.Name = "Sheet"
       ActiveSheet.Protect Password:="xyz", DrawingObjects:=True, Contents:=True, Scenarios:=True
    End If
    ThisWorkbook.Protect Password:="xyz"
End Sub

Thanks,
Sandu