The mapping definition cannot be nested under a type [_doc] unless include_type_name is set to true.

This error message typically occurs when trying to create a mapping for Elasticsearch 7.x or later, which requires the use of the `_doc` type instead of explicit types like `tweet` or `user`. To resolve this error, you can either set `include_type_name` to `true` in your mapping definition, or remove any references to explicit types and use `_doc` as the default type.

For example, if you were previously using a mapping like this:

PUT my_index
{
  "mappings": {
    "tweet": {
      "properties": {
        "message": {
          "type": "text"
        }
      }
    }
  }
}

You would need to update it to use `_doc` instead:

PUT my_index
{
  "mappings": {
    "_doc": {
      "properties": {
        "message": {
          "type": "text"
        }
      }
    }
  }
}

Alternatively, you could add `include_type_name: true` to your mapping definition:

PUT my_index
{
  "mappings": {
    "include_type_name": true,
    "tweet": {
      "properties": {
        "message": {
          "type": "text"
        }
      }
    }
  }
}