Unable to change my subscription

I am trying to change my subscription from Basic to Free because the project that needed answers have finished.
I can go all the way through selecting the Free subscription and answering all the questions, but when I arrive on the final confirmation page, the Confirm button it deactivated and nothing seems to be able to change that.
Any idea why this is - I need to cancel within 5 days or a new month’s payment is drawn from my bank.