Please help with my name servers

[boot]

Please Help,
when updating my domain nameservers to point to the server as advised in email 3/3.

Primary Nameserver: ns1.xf-s.com
Secondary Nameserver: ns2.xf-s.com
At
I get;

"Name server ns2.xf-s.com does not exist!"
:confused:
Thanks in Advance.