Private Sub Workbook_BeforeClose(Cancel As Boolean)
Dim lastRow As Long
Dim ws As Worksheet

Set ws = ThisWorkbook.Worksheets("Sheet1") ' Change this to the sheet you need to check!
lastRow = ws.Range("B" & Rows.Count).End(xlUp).Row

Dim rng As Range, cel As Range
Set rng = ws.Range(ws.Cells(1, 2), ws.Cells(lastRow, 2))

For Each cel In rng
    If cel.Offset(0, -1).Value = "" And cel.Value <> "" Then
        MsgBox (cel.Address & " is empty. Please populate before closing file.")
        cel.Offset(0, -1).Interior.Color = RGB(255, 0, 0)
        Cancel = True
        ' Exit Sub
    End If
Next cel

End Sub