Getting ORA-00947: not enough values error while inserting date into table
You are only inserting one column value, not both. The second parameter in to_date should be the format used by the first parameter, not another date. Your insert should look like this:

insert into times (departure, arrival) values 
  (to_date('2018-05-02 12:40','YYYY-MM-DD HH24:MI'),
   to_date('2018-05-02 15:08','YYYY-MM-DD HH24:MI'));