I can’t log into my existing paramount plus account with my apple tv or my iPhone

If you have a Paramount+ subscription from Paramount directly, on a Paramount+ account, then you’ll need to install an use the Paramount+ app on your devices. A subscription started outside the Apple TV app is not valid for the Apple TV app Channels of the same name, unfortunately. These are different types of subscriptions, with different terms and conditions, and different perks and limitations, and are not exchangeable as such.