The "Code was already redeemed" error message is coming from google authentication and not related to CPM's PIN system.
Try the following:
- Make sure you are using a supported browser such as Chrome, Safari, or Firefox.
- Clear your cache.
- Make sure you are not using parental control software or adblocker, which can disrupt network communications.
You could also try logging on to a different device which usually solves the situation.