Solved
Unable to view STARZ through my Roku TV
I have starz app on my Roku TV but it won’t let me log into my STARZ on my smart TV
Assuming your "smart TV" a non-Roku TV, then it sounds like you subscribed to Starz via Roku Pay, rather than subscribing to Starz directly.
If you subscribe to a service via Roku Pay (billing through Roku), then those are generally limited to viewing via Roku.
If that's the case, then cancel Starz via Roku Pay, and then subscribe directly, using those credentials to sign in to the Starz app on all devices (Roku and any other platform).
