Good morning!!

I have an excel document that I would like to have mandatory cells in a specific sheet with specific cells. Below is what I've come up with.. but it does not work. The range is the cells that I need to be mandatory. Ideally I would like to have separate VBA code separate to denote in the messages box what is missing rater then it say Please field out required fields. Any help would be great!! Thank you in advance.

Private Sub Workbook_BeforeClose(Cancel As Boolean)
If Sheets("Sheet1").Range("B7,B8,B9,B10,B11,B13,B14,B15,B16,B18,B19,B20").Value = "" Then
MsgBox "Please field out required fields!"
Cancel = True 'cancels the save event
Exit Sub
End If


End Sub