.EU and .DE domains register their nameservers differently then most other domains. To do this, you do not actually use the regular “Register a nameserver tool” in ENOM.
Example Domain: domain.eu
Example IP Address: 192.168.1.1
Once logged into your enom account, click on domains then My Domains. On this page, search for the domain in question and click on the domain name.
192.168.1.1--ns1.domain.eu
192.168.1.1--ns2.domain.eu
Once you have entered them like this the nameservers will be registered, and you can now use them for other domains as well.