But did you bought The Grand 3 SE at that time or it was only a demo? The code that came with Cubase is only for demo and can’t be used to upgrade to full version.
So my question is:
Did you bought The Grand SE 3? Do you remember?
P.S. When you buy the license for The Grand 3 SE, in fact you will receive another code (full). That demo code will expire as expected.