Do you actually have the DNS zone for the primary domain added to your server?
You need the zone file for the primary domain you will be using added to your server.
The "add an a entry for this nameserver" just picks an open IP from the server's IP and assigns it to the zone file but you need to first have the base domain zone file added to the server else it will only create new zone files with the DNS hostname subdomain name and not the base domain zone.
Ideally, it's often easier to just edit the zone file after it's added to your server either as a new cpanel account or just as an added DNS domain name on the server. Editing the DNS zone, you would need to set the DNS hostname at the top of the DNS zone file, the NS records, and the A records for each DNS server respectively.
/etc/nameserverips should also have your DNS servers listed as well:
Other than that, you need to have the same information registered with the registrar where you purchased your domain creating NS1 and NS2 dns hosts using the same respective IPs as you assigned.