This issue is most commonly related to organizational firewalls. Please ensure edx-for-business-no-reply@info.edx.org is whitelisted by your organization and ask the learner to check their spam and "other" folders.
Try sending them a reminder email. If they have not received the assignment or a reminder email after 24 hours, please reach out to customersuccess@edx.org with the email of the learner, the approximate date and time the code was assigned, and the date and time the reminder was sent.