operation illegal on system defined metadata

  Explanation:  You attempted to  define,  delete,  or  change  an
  index,  constraint,  or trigger on one or more Oracle Rdb system
  relations.  Or you attempted to drop  or  alter  an  Oracle  Rdb
  system  sequence.   Such changes, if permitted, could render the
  database unusable.

  User Action:  Do not attempt any of these modifications  to  the
  system relations or system sequences.