SQL> alter user scott account unlock identified by 123456;
alter user scott account unlock identified by 123456
*
ERROR at line 1:
ORA-00988: missing or invalid password(s)
SQL> alter user scott account unlock identified by "123456";
User altered.
SQL> conn scott/123456
Connected.
SQL> select table_name from user_tables;
TABLE_NAME
------------------------------
DEPT
EMP
BONUS
SALGRADE