Paramount Plus is available on Roku, but it is a paid subscription streaming channel. To watch paramount plus free, you can subscribe to any streaming services such as Philo TV, Fubo TV, Sling TV, Vidgo Plus, and YouTube TV, and these services provider five days free trial or seven days free trial.

Turn on your Roku device and TV.
Select the Paramount Plus app on the home page. And Click the Add channel button.
On your PC or smartphone, visit the website.
Enter the activation code displays on your screen. Then, click the activate button.
Finally, log in to your account, and you can start to stream the video contents.
By following these steps I was able to activate paramount plus on my roku. You can also try this.