Upload failed - Unknown username error when submitting pre-checkin code review

If you see this error message when setting up a pre-checkin code review with kwcodereview: Upload failed: Unknown username '<user_name>' This means that the reviewer you attempted to invite to your code review is not known to Code Review. The reviewer needs to log in to Code Review before you can invite them to your code review. See Participating in code reviews for details on how to log in to Code Review.