Welcome to the Roku Community and we appreciate you for reaching us.
As a first step, you can try removing the channel from the Roku home screen by navigating to the channel tile, pressing the * key on your remote, and choosing 'Remove channel'. Then restart your device from Settings>System>System restart. Once your player starts up again, add the channel back once more and try to sign in/up again.
If the issue still persists, it's best to reach out to the channel provider directly to get the channel going on your Roku. They can resolve the error you are having with the channel login. Many channels on Roku are developed and maintained by the channel provider themselves.
Please keep us posted.