An INDEX ONLY index can only be created on an empty table

  Explanation:  You attempted to create an INDEX ONLY index  on  a
  table  which already contains data.  This operation is currently
  restricted.

  User Action:  Either create a sorted or hashed  secondary  index
  on  the  table,  or  unload  the data from the table so that the
  INDEX ONLY index can be created.