Please make sure you have approved issuance of the certificate by following link in approval email. Check your inbox for the delivery of approval email in case if you have not approved the issuance yet. Do not forget to check your junk/bulk/spam folder.
You should follow instructions provided in What should I do if approval email was not delivered? article if no email has been delivered.
If you are using the HTTP or DNS validation, go to the certificate details page >> Edit methods page >> click Edit methods >> click Save changes/Retry Alt DCV.
If you have already approved the issuance, please review articles from Validation category for more information.