ALTER is not allowed on an INDEX ONLY index

  Explanation:  You have attempted to alter an INDEX  ONLY  index.
  This operation is not allowed.

  User Action:  Re-evaluate why this change is  necessary  on  the
  INDEX  ONLY  index.   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
  convert the INDEX ONLY table into a regular table.  The user can
  then alter the table, and recreate the INDEX ONLY index.