PHP卷曲代码400 [API]

I am trying to create the product via selly.gg API and receive the following error:

400 {"message":"Validation failed","errors":["Theme is not included in the list"]}

API documentation link

What I've already tried: 100 times edit my json, copy pasted it exactly from documentation and etc, tried a lot of connect methods, already read a lot of resources for a find a mistake by myself... but nothing.

My current code:

<?php

$URL='https://selly.gg/api/v2/products';

$data = [
    'title' => 'test title',
    'description'=> 'test description',
    'stock' => '1',
    'private' => 'false',
    "unlisted" => 'true',
    "seller_note" => 'test seller note',
    'price' => '9.99',
    'currency' => 'USD',
    'gateways' => ['paypal'],
    'product_type' => '2',
    'info' => 'FirstKey, SecondKey, ThirdKey'
];

$data_json = json_encode($data);

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $URL);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    "Authorization: Basic " . base64_encode("mega4642179@gmail.com:3pnAUztziYgZDaAbNjYSuWSeNhtCjcfET8XLFxe8FH2W2kaoBg"),
    'Content-Type: application/json',
    'Content-Length: ' . strlen($data_json)
));
curl_setopt($ch, CURLOPT_TIMEOUT, 30);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_json);
curl_setopt($ch, CURLOPT_USERAGENT, "SellyDirection - selly.loc");
$result = curl_exec($ch);
$status_code = curl_getinfo($ch, CURLINFO_HTTP_CODE);

// Display errors
echo $status_code;
echo $result;
curl_close ($ch);

?>