This error has three common causes:
1.The realm is misspelled, e.g. pbh@AHTENA.MIT.EDU instead of pbh@ATHENA.MIT.EDU (realms are case sensitive).
2.Your krb.con file contains an entry for ATHENA.MIT.EDU but not athena.mit.edu.
3.The realm is missing from your KRB.CON file, which should be located in your \net\kerb directory. If you suspect the problem is with your KRB.CON file, either call the Network Help Desk, 3-4101, or copy the /etc/krb.conf file from a nearby UNIX workstation to your \net\kerb\krb.con file.