I had opened a ticket and got an answer today. Enom also could not register the nameservers so they pushed me to CentralNic, who in turn went to their source. They say the nameservers have now been registered and I am waiting to confirm. Oddly, Enom will not allow me to point the domain name to those nameservers yet, maybe after they propagate? Who knows.
Thanks for the responses.
We have numerous domains in that pattern through eNom with never an error
You should open a Support ticket
marcbarak: were you able to use them as registered nameservers?
I have several domains registered through Enom like that as well, but only till we tried to create name servers with one did we find the problem.
After CentralNic asked their supplier(?) to create the nameservers ns1-ns2.mydomain.us.com for me, Enom still gave errors when trying to point any domain to those nameservers. I went to a Godaddy account I had and pointed a domian to ns1-ns2.mydomain.us.com and it worked fine, proving that the nameservers had been created.
CentralNic is working with Enom to resolve the issue.
Enom is a busy place, it can take 24-48 hrs to get a response from support.
Turned out to be a misconfigured DNS at CentralNic. They have fixed the problem and I can now use ns1.mydomain.us.com
All of the domain.us.com's were misconfigured when purchased at Enom.
As marcbarak said, you can use a subdomain.