I got the same email, except my offer was 10,000 points for 12 nights. I also got the error. This is not the first time I've gotten an email offering a promo but upon registration, it saying I'm not eligible. I sent an email off to IHG calling them out for this a second time.
The last time, they claimed the offer had expired a week before I get the email. I wasn't happy to hear that and they wouldn't cough up points or status for it. They better do something this time, or I'll stop staying at IHG whenever possible.