Thanks for reaching out here in Roku Community.
I understand that you are trying to access your Showtime subscription on The Roku Channel website, is that correct? Have you made sure that you have a subscription to the channel through The Roku Channel? You can confirm this by going under Manage your subscriptions in your Roku account, it'll indicate if the subscription is through The Roku Channel.
Please be noted that existing subscriptions from Showtime Standalone cannot be used to view premium content on The Roku Channel through a Roku device or Roku website.
Keep us posted on what you find out.