Double-check to ensure the e-mail and phone number were entered correctly. If the e-mail was entered incorrectly, cancel the request and send a new one. If the phone number was entered incorrectly, simply edit the request and click "Save" which will automatically send an updated request.
If all information was entered correctly, their email provider may have blocked the CertifID email from making it through to their primary inbox. Please have them check their spam folders or any sub-folders where they could be stuck. Reach out to firstname.lastname@example.org.