Have you subscribed to HBO through the channel directly in addition to subscribing through Roku Pay?
For account and billing related issues regarding Roku Pay, please reach out to our support team directly here for your 'Account or billing' issue, here: https://support.roku.com/contactus. They will be able further assist you.
For more information about your subscription through HBO directly, you will need to contact HBO Support. You can reach them here: https://help.hbonow.com/