ALTER is not allowed on an INDEX ONLY table

  Explanation:  You have attempted to alter an INDEX  ONLY  table.
  When  an  INDEX  ONLY  index is defined on a table, the table is
  then labeled  an  INDEX  ONLY  table.   This  operation  is  not
  allowed.

  User Action:  Re-evaluate why this change is  necessary  on  the
  INDEX  ONLY  table.   If  the  change is necessary then drop the
  INDEX ONLY index which will move the data from  the  index  into
  the  table.   The  action  of dropping the INDEX ONLY index will
  cause the the INDEX ONLY table to revert to regular  table,  and
  the alter table operation will be allowed.