@Rahmon did you test it with Elasticsearch 5.6.16 (the version shipped with WP Local Docker)? I'm using the following snippet in theme's header.php:

new WP_User_Query(
	[
		'search'  => 'test',
		'orderby' => 'display_name',
	]
);

As is, I receive a 400 error saying:

Fielddata is disabled on text fields by default. Set fielddata=true on [display_name.sortable] in order to load fielddata in memory by uninverting the inverted index. Note that this can however use significant memory. Alternatively use a keyword field instead.

If I change wp-content/plugins/elasticpress/includes/mappings/user/initial.php to look like this, it works:

'display_name'    => array(
	'type'   => 'text',
	'fields' => array(
		'raw'      => array(
			'type'                => 'keyword',
			'ignore_above' => 10922,
		),
		'sortable' => array(
			'type'         => 'keyword',
			'ignore_above' => 10922,
			'normalizer'   => 'lowerasciinormalizer',
		),
	),
),

Can you please check it? Thanks

@Rahmon did you test it with Elasticsearch 5.6.16 (the version shipped with WP Local Docker)? I'm using the following snippet in theme's header.php:

new WP_User_Query(
	[
		'search'  => 'test',
		'orderby' => 'display_name',
	]
);

As is, I receive a 400 error saying:

Fielddata is disabled on text fields by default. Set fielddata=true on [display_name.sortable] in order to load fielddata in memory by uninverting the inverted index. Note that this can however use significant memory. Alternatively use a keyword field instead.

If I change wp-content/plugins/elasticpress/includes/mappings/user/initial.php to look like this, it works:

'display_name'    => array(
	'type'   => 'text',
	'fields' => array(
		'raw'      => array(
			'type'                => 'keyword',
			'ignore_above' => 10922,
		),
		'sortable' => array(
			'type'         => 'keyword',
			'ignore_above' => 10922,
			'normalizer'   => 'lowerasciinormalizer',
		),
	),
),

Can you please check it? Thanks

@felipeelia I've updated the mapping and updated some tests to check if the query uses the Elasticsearch to retrieve the users.