I face with that error while try to set SQL Plan on one of my sql.

This is the one of he published bug.

Bug 10313110  SQL Advisor raises an ORA-13786 when Trying to Implement a Recommended Parallel Profile

If I try to use below syntax its also through error message such as:

Check the current plan advise:

SET LONG 10000000;
 COLUMN RECOMMENDATIONS FORMAT A200 WORD_WRAPPED
 SET pagesize 0
 SET LONG 10000000
 SET feed OFF
 SET TRIMSPOOL ON
 SET linesize 200
 SELECT DBMS_SQLTUNE.REPORT_TUNING_TASK('&TASK_NAME') AS RECOMMENDATIONS FROM DUAL;
Enter value for task_name: SQL_TUNING_123456789
 old   1: SELECT DBMS_SQLTUNE.REPORT_TUNING_TASK('&TASK_NAME') AS RECOMMENDATIONS
 new   1: SELECT DBMS_SQLTUNE.REPORT_TUNING_TASK('SQL_TUNING_1606725746461') AS RECOMMENDATIONS

And then run below 

DECLARE
   l_sql_tune_task_id  VARCHAR2(200);
 BEGIN
   l_sql_tune_task_id := DBMS_SQLTUNE.accept_sql_profile (
                           task_name => 'SQL_TUNING_1606725746461',
                           force_match  => TRUE,
                           profile_type => DBMS_SQLTUNE.PX_PROFILE);
 END;
 /

It may throw below error:

ERROR at line 1:  ORA-13786: missing SQL text of statement object "1" for tuning task "SQL_TUNING_1606725746461"
 ORA-06512: at "SYS.DBMS_SYS_ERROR", line 95
 ORA-06512: at "SYS.DBMS_SQLTUNE_INTERNAL", line 16442
 ORA-06512: at "SYS.PRVT_SQLPROF_INFRA", line 31
 ORA-06512: at "SYS.DBMS_SQLTUNE", line 7544
 ORA-06512: at line 4

So please try to set your sql plan by using below syntax:

SQL> execute dbms_sqltune.accept_sql_profile(task_name =>'NEW_PLAN', task_owner => 'ABC ',replace => TRUE);