Can you please clarify what you mean when you "rebooted your Roku"? Rebooting typically refers to powering the device off and then back on again. Did you perform a factory reset on your Roku device instead? Can you specify what is being asked on screen? Can you provide a screenshot? How are you accessing the Showtime content? Are you using Showtime within The Roku Channel, or are you accessing Showtime through the channel directly?
With more detailed information, we will be able to assist you further.