For sites using the WP Discourse plugin, with WordPress set as the SSO provider, there can occasionally be issues with users attempting to login to existing Discourse accounts when their email address has not been validated by WordPress. The most common cause of this is when a user with an existing account on Discourse creates a WordPress account after you have activated the WP Discourse plugin. This should only ever happen on sites that are not using the default WordPress registration system. You’ll know it is happening if you get reports from users that they receive this error message when attempting to login to Discourse with SSO:
To fix the issue, first confirm that the user can respond to an email sent to their WordPress email address. This is to ensure that the user isn’t attempting to take over a Discourse account. Then, logged into your WordPress admin account, go to the user’s profile page and scroll to the Discourse section. Check the ‘Email Address Verified’ checkbox to mark the user’s email as valid. After saving the updated profile, the user should be able to login to Discourse.
For more details about SSO login issues, see Debugging and fixing common SSO issues.