After configuring a custom domain and verifying the correct CNAME is used, the verification on the custom domain does not finish.
If the custom domain is tested multiple times, it can hit the Duplicate Certificate Limit: Duplicate Certificate Limit - Let's Encrypt
If an Auth0 managed custom domain is deleted and created multiple times within a short period, it can hit the maximum number of certificates that can be issued. The same can occur if multiple tenants are configured around the same time with the same parent domain.
Provided that the CNAME is correct and in place, the certificate should be allocated within a couple of days. Using a command like the following, the status of the certificate can be checked.
openssl s_client -servername <YOUR_CUSTOM_DOMAIN> -connect <YOUR_CUSTOM_DOMAIN>:443
Once a valid certificate shows on the response for the domain, pressing the verify button should complete the verification.
Contact the Support Center if you continue to have issues with the verification of the custom domain.