Explanation:
There is no user with this name.
User Action:
A DBA or the SYSDBA can use a <QUERY statement> issued on the system table DOMAIN.USERS to find out the known user names. Other users can use a <QUERY statement> issued on the system table DOMAIN.USERS to find out the users which belong to the same DBA as they do. Repeat the SQL statement with a known user name.