Tried to change an user's attribute, but it failed with ORA-01918.
SQL> alter user ht account unlock;
alter user ht account unlock
ERROR at line 1:
ORA-01918: user 'HT' does not exist
ORA-01918 means that the username you provided in ALTER USER statement is invalid, please choose a valid one or correct it.
To make sure which username should be use, you may query DBA_USERS for sure.
In this case, we have a typo in the previous statement, we correct it like this:
SQL> alter user hr account unlock;
We fixed it.
To know whether the user is a predefined user or not, you may check DEFAULT_PWD$.