An INDEX ONLY index must be the only index defined on the table

  Explanation:  You attempted to create an INDEX ONLY index  on  a
  table  which  already  had indices defined upon the table.  This
  operation is currently restricted.

  User Action:  Either use a sorted or hashed secondary  index  on
  the  table,  or  delete  the  currently  defined indices on this
  table, so that the INDEX ONLY index can be created.