ORA-12838: cannot read/modify an object after modi

insert data insert being given:

SCOTT@prod>insert /*+ append */ into scott.employee select * from scott.employee ;
3584 rows created.
SCOTT@prod>/
insert /*+ append */ into scott.employee select * from scott.employee
                                *
ERROR at line 1:
ORA-12838: cannot read/modify an object after modifying it in parallel

View table parallelism

SQL> select a.degree from dba_tables a where a.table_name='EMPLOYEE';
DEGREE
-----------
1

View lock information

SELECT o.object_name,
       o.owner,
       l.locked_mode,
       s.sid,
       s.serial#,
       s.logon_time
  FROM v$locked_object l, dba_objects o, v$session s
 WHERE l.object_id　 = o.object_id
   AND l.session_id = s.sid
   AND o.object_name = 'EMPLOYEE';
   
OBJECT_NAME      OWNER   LOCKED_MODE        SID    SERIAL# LOGON_TIME
---------------- ------- ----------- ---------- ---------- -----------
EMPLOYEE         SCOTT             6         80       5328 2019/11/24

ora-12838 error

SCOTT@prod>!oerr ora 12838
12838, 00000, "cannot read/modify an object after modifying it in parallel"
// *Cause: Within the same transaction, an attempt was made to add read or 
// modification statements on a table after it had been modified in parallel
// or with direct load. This is not permitted.
// *Action: Rewrite the transaction, or break it up into two transactions:
// one containing the initial modification and the second containing the
// parallel modification operation.

append inserted on the high-level data and generating less redo data, plus rapid insertion speed will also holds an exclusive lock, for the transaction performed after uncommitted easily lead to the problem.

MOS related documents

ORA-12838 with Direct Load Inserts (文档 ID 116494.1)
PARALLEL RESTRICTIONS:A transaction can contain multiple parallel DML statements that modilydifferent tables, but after a parallel DML statement modifies a table,
NO subsequent serial or Parallel statment (DML or QUERY) can access thesame table again in that transaction.***Each insert in SQL*plus is considered a transaction, 
while the wholePL/SQL BLOCK is considered a transation.