If you subscribe to something through Roku you can often only use the subscription on an actual Roku device. The Roku app on another device isn't a substitute for an actual Roku device.
Having said that, I can't say positively that that's your problem, but I'm pretty sure you can't use a "premium" service on the Roku app on another platform. The fine print when you subscribe states, "Certain content may be viewed only within The Roku Channel, only on Roku devices, or only in certain geographic locations – see offer terms for details." I didn't get too far looking at the offer terms for more specifics. I've never used the Roku app on some other device, so I don't know what it offers.
If you really want to try out Paramount+ via Roku, you can add a Roku device to your FireTV. By the way, you don't need to subscribe to anything through Roku. If you already have a Paramount+ subscription directly with them, you could use that subscription on a Roku device. However, if your subscription is through Amazon, you probably can't (they often have the same limitations trying to use their subscriptions on other devices). It's usually recommended to take out a subscription directly with the provider if you want to be able to use that subscription anywhere.
