If you want change IP nameserver on .EU domain or use subordinate nameserver you need to use format nameserver ip
For example, if you want use nameservers ns1.customer.seflow.it and ns2.customer.seflow.it on domain seflow.eu, you will see error:
"ns to add is subordinate but has no ip addresses"
To avoid that please use format
Remember to replace ''hostname and ip with your nameservers host and ips.