constraint <str> can only be deleted  by  changing  or  deleting
  relation <str>

  Explanation:   The  constraint  specified  was  defined   as   a
  relation-specific  constraint,  and can only be deleted by using
  the CHANGE RELATION or DELETE RELATION statement on the relation
  declaring the constraint.

  User Action:   First,  review  the  database  design;  then,  if
  desired, delete the relation-specific constraint.