Whicg browser do you use? The buying page is hosted by points.com. They have different safety measures than IHG, e.g. in regard to IP adresses, VPN. I tried around and e.g. failed to access the page while using the WiFi of an airport lounge (AA Club in Charlotte) and also using my VPN sometimes failed. I also fail if I use he VPN setting of Opera.

I also discovered some problems from countries in certain regions.

You might try to contact points.com instead