If your subscription was made directly with Paramount on Paramount's website, you should be able to log into it via Paramount's own channel app installed to your Roku. Don't try to access and log in from within The Roku Channel - your account is with Paramount, not Roku, so The Roku Channel will know nothing about it.
The initial Paramount+ page should look like this:
You should be able to SIGN IN from there.
If you choose the log in on your TV option, it will offer to use the email address associated with your Roku account:
If you used a different email address for Paramount than you use with Roku, click "Use different email" instead of "Continue". Then log in with your appropriate Paramount credentials.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X