I recently had the same type of thing happen.
I ordered 6 gift cards last week. (4) $500 and (2) $100 cards.
I received emails for each (e-cards).
All worked except one $500 card was listed as invalid.
I looked up the card balance at another site and it came up invalid also.
The other cards showed the balance, or having a zero balance after they were used.
I then contacted AARP and they took my info and said that it would be resolved within 3-5 business days.
Here it is 6 business days, no contact so I contacted AARP again.
They said that it is in another group and they will contact me within 3-5 business days.
Luckily I still have a couple more weeks until the cruise needs to be paid off.
I can understand that something went wrong with the ordering, but I really wish that AARP would get this resolved faster. To me it's a no brainer, since it says it is invalid, not used.