modifie le code comme suit :
Sub Onglet_S()
Dim aa As String, i As Variant, j As Integer, k As String, l
For Each i In Sheets
aa = i.Name
For j = 1 To Len(aa)
k = Mid(aa, j, 1)
l = Mid(aa, j + 1, 2)
If UCase(k) = "S" And IsNumeric(l) = True Then
i.Range("A1") = "Le nom de cet onglet contient S" & l
End If
Next j
Next i
End Sub