Upgrading to peacock premium plus

I recently enrolled in the 1 gig internet plan with peacock premium. I tried to upgrade the plan to peacock premium plus via the stream store. I add it to my cart, review all the billing details but when I click submit order I receive a message "Sorry, looks like you will need help completing this order. Please call or use Xfinity assistant" I tried the chat assistant but they are very unhelpful. I just tried the phone number and they were worse. I understand very clearly how to upgrade the plan, they make it very easy to do online. I am just not able to submit my order and no one can help me. This is very frustrating. I am aware that I can use a different email to purchase peacock premium plus separately on their website--but then why would Xfinity have this option available in their stream store if it does not let the customer complete the order?