Registrar conflict with the DNS provider
When you register the domain with one provider, but point the name servers to another provider, then the CNAME/TXT/MX/SPF records added in your domain registrar are not considered valid. This is because you might have changed the DNS provider (either accidentally or for hosting your website), or your previous email provider configuration.
The CNAME/TXT records added in the provider where the name servers are pointed will be effective and valid. To check this, do one of the following:
Do a 'NameServer' lookup for your domain to check where your domain is hosted.
Check with your domain registrar or the technical contact for your domain to know where the name servers are pointed.
If you have a large TTL value (24 hrs/48 hrs), then the CNAME/TXT/MX records you have added in your DNS provider might not be provided during the verification process.
Check the TTL value and try verifying after some time.
The way the CNAME/MX needs to be added varies with different DNS providers. Some DNS providers expect you to add the domain name in the hostname (zb*****.<yourdomain.com>), whereas others expect you to just fill the hostname (zb*****), which is appended later.
Check the help guides and instruction manuals or reach out to the support team of your DNS provider.
Wrong domain name
You might have typed the wrong domain name.
Check if you have added the correct domain name. If not, delete the domain and add it again, after which a new zbcode will be generated.
Add a new CNAME or TXT record in your DNS provider with the newly-generated zbcode for verification.
When entering the CNAME or TXT record value, do not add a dot or space at the end of the value as it may get affected.