relation  constraint  <str>  is  referenced  by  relation  <str>
  constraint <str>

  Explanation:   The  UNIQUE  or  PRIMARY  KEY   relation-specific
  constraint  that you tried to delete with the DELETE RELATION or
  CHANGE RELATION statement cannot be deleted.  You cannot  delete
  the  UNIQUE  or PRIMARY KEY relation-specific constraint because
  it  is  referenced  by  one  or  more  FOREIGN  KEY   REFERENCES
  relation-specific  constraints  from  one  or  more  relation or
  relations.

  User Action:   First,  review  the  database  design;  then,  if
  desired,  delete  the  FOREIGN  KEY REFERENCES relation-specific
  constraint (or constraints) and try the operation again.