@LS098, if you subscribe to something through Roku you never have to sign in, so something else is going on. There's often multiple ways to subscribe to different services and if you subscribe to something through The Roku Channel then you can only watch through The Roku Channel. Just a guess, but you may have subscribed through The Roku Channel and then tried to use that subscription on the stand-alone channel/app (or vice versa).
You will find that most people here would suggest that you only subscribe to a service directly through the provider and avoid any middleman like Roku, Amazon, Apple, etc.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.