Welcome to Roku Community.
We understand that you are experiencing difficulties logging in to the Sling TV channel. We will assist you in resolving this issue. As a troubleshooting step, you could try removing the channel, restarting your device, reinstalling the channel, and attempting to sign in again to see if this resolves the issue.
If the issue persists after trying the steps, the username and password associated with your subscription are managed directly with your channel provider. Please be advised that channel logins or authentication are handled directly by the channel developer. We suggest reaching out to Sling TV support to report the issue and for further help. They can resolve the error you are having with channel authentication.
We aim to resolve this matter for you. Please don't hesitate to contact us if you have any other issues or inquiries. We are always here to assist you.