illegal thresholds usage - area <str> is MIXED format,  must  be
  UNIFORM

  Explanation:  Thresholds in storage maps can only be applied  to
  UNIFORM areas.

  User Action:  Reference UNIFORM areas if you wish to use storage
  map  thresholds.  Otherwise, define MIXED areas thresholds using
  CREATE STORAGE AREA  or  DEFINE  STORAGE  AREA  statements  when
  creating or altering the database.