Reindex requires _source to be enabled for all documents in the source index. You must also set up the destination index before calling _reindex.
Reindex does not copy the settings from the source index. Mappings, shard counts, replicas, and so on must be configured ahead of time.
Here is a simple example, you just need to change my-index & my-index-aux with your indexes names.
GET /my-index/_mapping