ORA-14400: inserted partition key does not map to any partition
It is an older article and it's assuming the year is 2015.
Change the script so partitions look like this:

PARTITION BY RANGE (created_Date)
(PARTITION tab1_part_2017 VALUES LESS THAN (TO_DATE('01/01/2018', 'DD/MM/YYYY')) TABLESPACE users,
 PARTITION tab1_part_2018 VALUES LESS THAN (TO_DATE('01/01/2019', 'DD/MM/YYYY')) TABLESPACE users);


The whole script should look like this:

DROP TABLE tab1 PURGE;

CREATE TABLE tab1 (
  id                NUMBER,
  code              VARCHAR2(20),
  description       VARCHAR2(50),
  created_date      DATE,
  CONSTRAINT tab1_pk PRIMARY KEY (id)
)
PARTITION BY RANGE (created_Date)
(PARTITION tab1_part_2017 VALUES LESS THAN (TO_DATE('01/01/2018', 'DD/MM/YYYY')) TABLESPACE users,
 PARTITION tab1_part_2018 VALUES LESS THAN (TO_DATE('01/01/2019', 'DD/MM/YYYY')) TABLESPACE users);

CREATE INDEX tab1_code_idx ON tab1(code) LOCAL;

INSERT INTO tab1
SELECT level,
       CASE
         WHEN MOD(level,2)=0 THEN 'CODE1'
         ELSE 'CODE2'
       END,
       CASE
         WHEN MOD(level,2)=0 THEN 'Description for CODE1'
         ELSE 'Description for CODE2'
       END,
       CASE
         WHEN MOD(level,2)=0 THEN SYSDATE
         ELSE ADD_MONTHS(SYSDATE, 12)
       END
FROM   dual
CONNECT BY level &lt;= 100000;
COMMIT;

EXEC DBMS_STATS.gather_table_stats(USER, 'TAB1');


This is because of this:

         WHEN MOD(level,2)=0 THEN SYSDATE
         ELSE ADD_MONTHS(SYSDATE, 12)


SYSDATE will be inserted into tab1_part_2017 and ADD_MONTHS(SYSDATE, 12) will be inserted into tab1_part_2018 and you will get equal number of rows in both partitions (50000 in each to be exact).