The most common reason an email domain doesn't verify is if the DNS provider automatically adds the main part of the domain to the entry you add into the service and as a result the domain is duplicated. For example if the entry to be added is "2021042933pm._domainkey.example.com" and you enter this as is into your dns provider, then the resulting entry will be "2021042933pm._domainkey.example.com.example.com".
If your DNS server only requires the prefix (or subdomain) to be entered then entering "2021042933pm._domainkey" will result in the desired entry.