you need to use the code from the Preview and Publish page
first you see a Channel Summary , then something like "Use the access code below to preview your unpublished channel on a Roku device. We strongly recommend previewing your channel prior to publishing." use the code from that part.
if that code is not working , and it is an unpublished channel - well delete it and create it again , nothing to loose.
Building a TV Channel can be easy … if you have a right tools ===>> https://tv-channel.info/