form_params и body - это разные параметры в жрете. проверьте json
$json = [
"filter" => json_encode((object)[]),
"skip" => 0,
"limit" => 20,
"sort" => json_encode((object)['createAt'=>1]),
"select" => "fullname username",
"populate" => "'right', 'group'"
];
$response = $client->request('get', 'http://sa-vn.com:2020/api/users/user', [
'json' => $json,
]);
Если возникнет какая-либо ошибка, попробуйте также без json_encode.
$json = [
"filter" => (object)[],
"skip" => 0,
"limit" => 20,
"sort" => (object)['createAt'=>1],
"select" => "fullname username",
"populate" => "'right', 'group'"
];