When i tried the ap it also said "reservation not found". I have a 2 last names and I tried with/without the hyphen, it finally worked with I tried my last names without a space, weird. (official ncl account is with a space)
Also I actually typed in the reservation number rather than cut and pasted it in. Try a few options ans see if it works.
The offer didn't show up in my AMEX offers either. I called yesterday and they we're going to call the marketing office, to see if I could get it. Not sure if I will, but at least AMEX put a note on my account about it. Call AMEX worst case scenario you haven't lost anything 🙂 Good luck