Upgrade checkout process doesn't go through

Trying to upgrade my internet plan as an existing customer through the "Change Plan" option, I get an error as soon as click through from the Equipment stage to the Flex stage. The Flex page pops up for a second and then immediately redirects to https://www.xfinity.com/error/buy/sorry?errorCode=buy-error where it just says "Looks like something went wrong." I tried two different browsers just in case.

When I got on chat, the rep told me: "With regards on the package that includes Flex. It doesn't push through with our Xfinity Website." Not sure what this means. I don't have or want Flex but all the plans available to me initially include it by default I think. He then tried to go through a sales script to get me to upgrade right there. I'm extremely reluctant to do this through a representative because of the "misunderstandings" I've had over the years where I would end up with either higher charges or disruptions to my service. To the point where I might just keep my current plan. If I can't do this myself, I don't know why that "Change Plan" link is right there in my account.