DNS Provider - Registrar conflict
When you register the domain with one provider but change the Name Servers or host your DNS with another provider, the CNAME records added in your Domain Registrar are not considered valid. You may have changed the DNS Provider for hosting your website, your previous email provider configuration, or based on your choice. The CNAME records added in the DNS hosting provider will only be effective and valid. Hence, do a NameServer Lookup for your domain to check where your DNS is hosted. You may also check with your Domain Registrar or the technical contact for your domain on where the name servers are pointed if you are not sure.Longer TTL TTL (Time To Live) is the time specified in your DNS for each change in your DNS to take effect.
If you have a large TTL value (24 hrs / 48 hrs), the CNAME Records might not propagate during the verification process. It might take up to 12–24 hours for DNS changes to take effect based on the TTL set. Please check the TTL value and try verifying after a while.Incorrect Values
The way the CNAME needs to be added often varies with different DNS Providers. Some DNS Providers expect the domain name also to be added in the hostname (for example:sm **.yourdomain.com), whereas some expect you to fill only the hostname part (sm**) and append the domain automatically.
Hence, it is recommended to check the help pages or instruction manuals or reach out to the support team of your DNS provider to ensure the CNAME records are added correctly.