Select the session id and kill the process which is holding the lock on schema user.
select c.owner, c.object_name, c.object_type, b.sid, b.serial#, b.status,
b.osuser, b.machine from v$locked_object a, v$session b, dba_objects c
where b.sid = a.session_id and a.object_id = c.object_id;
and then
SQL> select inst_id,sid,serial# from gv$session where username='SCOTT';
INST_ID SID SERIAL#
---------- ---------- ----------
1 130 620
SQL> alter system kill session '130,620,1';
alter system kill session '130,620,1'
*
ERROR at line 1:
ORA-00026: missing or invalid session ID
Now, it works:
SQL> alter system kill session '130,620,@1';
System altered.