You can do that with code in the workbook's ThisWorkbook module, like:
Code:
Private Sub Workbook_BeforeClose(Cancel As Boolean)
With Sheets("Sheet1")
If .Range("B9").Value = "" Then Cancel = True
If .Range("B20").Value = "" Then Cancel = True
If .Range("G13").Value = "" Then Cancel = True
End With
If Cancel = True Then MsgBox "Please complete all required cells"
End Sub