Make sure you're not trying to use "Paramount+ on The Roku Channel". Assuming you're in the US, you should be able to add it from the website:
https://channelstore.roku.com/details/32b338eebb16d1de34870b6b1d674911/paramount-plus
You can also add it from "Streaming channels" on you Roku's Home Screen menu, but again, make sure you pick the right one. When you launch it, there should be a "Sign in" option.
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