These are the steps to take but as it is saying you will lose access to Prime after the 29th, it must have been cancelled already.
To cancel Amazon Prime for free, head to the ‘Setting’ section of ‘Your Account’. Then follow these steps:
Go to 'Manage Your Prime Membership'
To cancel a free trial, click 'Do not continue' - this is the second option on the left-hand side of the page
Note: If you select 'Do not continue' for your free trial, you'll still be able to receive the benefits until the final - 30th - day
On the final day, your membership will cancel, and your card won't be charged
Amazon says if you later change your mind and choose to continue your membership instead, you can revert your decision by clicking 'Continue membership' in 'Your Account'. You can opt out at any point during the free trial period.