Thanks for reaching out — let's check this out.
Is this the first time you're trying to link your Paramount+ account? What country are you located in? Also, do you have the latest version of the Google Home app? Try using a different mobile device to see if you'll get the same result.
I also have this problem and Google it periodically to see if there's any update, so I have tried on multiple phones and installs now. I'm using a Google Pixel 7 phone in the UK. My Home app is on 22.214.171.124. The Cast device firmware is on 1.56.275994. I know I'm not the OP just wanted to chime in so that it doesn't look like an isolated issue!