You need to have a Prime Video subscription to add the Paramount+ channel to the Prime video service. It doesn't work 100% of the time, just more often.
Sometimes, if I go into Paramount+ App, either on my Roku or FireTv, and instead of going to "Continue Watching", I go to CBS first, and navigate to the show I'm interested in watching, it works, but I think it's luck of the draw. If I pick the right show, at the right time, on the right day, pat my head, while rubbing my belly, and hop on one foot... It might work. Oh, I forget to mention, only if there is a full moon and all the stars align just right, I've got a chance to watch my show. I'm not always that lucky.