Andonny
An alternative to data validation is an event macro.
Right click your sheet name tab, left click View Code and paste this code into the white area. Use Alt+F11 to return to your sheet.
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
On Error Resume Next
If Target.Column = 2 Then
If Target.Offset(0, -1) = "" Then
MsgBox "You cannot enter data in Column B if the cell in Column A is blank"
Target.Offset(0, 1).Select
End If
End If
End Sub
You will get fewer unnecessary messages if you also go to Tools, Options, Edit and change the setting of "Move selection after Entry" so that the Direction is Right (rather than Down)
Good Luck
Derek