Please, note that you can not delete the card that is the only one (default one) linked to your workspace as well as you can not delete card that was charged last - in both cases you will see the error message “Can't delete last payment option”. 

If you need to change the card - add it as a new payment method and make it the default one (after that you will be able to delete all other cards). 

If you want to remove the card information before canceling your subscription, please note that your payment data can only be deleted along with all the other data associated with your workspace. In order to request that, submit a ticket here on our Helpdesk or drop an email to support@rocket.chat.