I have the Terabyte Internet Data Usage Plan and am attempting to add the Unlimited Data Option to my plan. According to the availability (https://www.xfinity.com/support/articles/data-usage-find-area?), I am eligible, as any customer with the Terabyte Plan can add the Unlimited Option. But when I add the plan (https://www.xfinity.com/buy/plan/checkout/XFINUDAT), I get the message: "We're sorry. It looks like this product isn't available for you to purchase right now. Please check back again soon." The icon for my cart shows that something was added to it, but when I click on the cart, I get an error message that simply says, "We're sorry." Why am I not able to add the Unlimited Option? Thanks.