Mandatory cells if another is filled

Try this...

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
    Dim ws As Worksheet, cell As Range
    For Each ws In Sheets(Array("JAN", "FEB", "MAR", "APR", "MAY", "JUN", "JUL", "AUG", "SEP", "OCT", "NOV", "DEC"))
        For Each cell In ws.Range("A2:A10")
            If Not IsEmpty(cell) And IsEmpty(cell.Offset(0, 7)) Then
                Application.Goto cell.Offset(0, 7)
                Cancel = True
                MsgBox "Reason required!"
                Exit Sub
            End If
        Next cell
    Next ws
End Sub

maybe something like this

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)

Dim rng As Range
Dim sh As Worksheet
Dim arrMonth As Variant
Dim i As Long

' Loop throu all the sheets in the workbook that have the name
' in the array
' Modify the array to add more months...
arrMonth = Array("JAN", "FEB", "MAR", "APR")

For i = 0 To UBound(arrMonth)
    Set rng = Worksheets(arrMonth(n)).Range("A2:A10")
    
    ' Loop through everty range in the sheets to see if there is a match.
    For Each cell In rng
        If Not IsEmpty(cell) And IsEmpty(cell.Offset(0, 7)) Then
            cell.Parent.Activate
            cell.Offset(0, 7).Select
            Cancel = True
            MsgBox "Reason required!"
        End If
    Next
Next i

End Sub

Thanks