With selection change event, something like below.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

If Target.Count 1 Then Exit Sub

If Intersect(Target,
Me.Range("A12,A14,A16,A18,A20,A22,A24,E12,E14,E16, E18,E20,E22,E24,K12,K14,K16,K18,K20,K22,K24,Q12,Q1 4,Q16,Q20,Q22,Q24"))
Is Nothing Then Exit Sub

If Len(Target) = 0 Then
Target.Value = "x"
Exit Sub
Else
Target.Value = ""
End If

End Sub