Why could registration reminders not be sent?

When I go to use the ‘re-send registrations’ feature on the select sample page, I see the red ‘registrations could not be sent’ message

If you see the red ‘registrations could not be sent’ message when you attempt to resend registration emails, this usually means that the accounts you are trying to resend the registration emails to have already logged into the platform. You will notice that they may have a ‘last login date’.
As the user has already logged in, you would instead need to send the user the main platform URL where they will be taken to the login page and will be able to login using the credentials they set. Alternatively, if they have forgotten their password you can send them the platforms' forgotten password link, which can be found on the login page.