Open Office, not sure what works compared to Excel.
Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Range("A1"), Target) Is Nothing Then Exit Sub
If Target.Value <> vbNullString And Target.Offset(0, 1).Value = vbNullString Then _
MsgBox ("Don't forget to enter the cell to the right.")
When cell A1 is changed and contains data and cell B1 doesn't, then a message is displayed.
Enter a Input Message (2nd tab) to show when cell is selected.
Right-click cell to add comment to cell.
Select B1 and choose to use a formula: =EN($A1<>"";$B1="")
This is what I could think of. Hopefully there is something you like and can use.