How do I create a product? How do I search for a product?

[Lollichkeit]

My problem has been closed, but my problem is still there:

My problem:
My goal is:
Reading out whether the product is available and if not, then creating it, but I already have an error while creating it.

My code:

...
$product = new Product;
$title = new TranslatableText;
$title->text = 'Test';
$title->languageCode = 'de';

$product->title 		= $title;
$product->sku		= 'test';
$product->soldAmount	=  99.99;

print_r($product);

$test = $client->createProduct($product);
print_r($test);
...

My Error:
Fatal error:
Uncaught GuzzleHttp\Exception\ClientException:
Client error: POST https://app.billbee.io/api/v1/products resulted in a 400 Bad Request response: {"Message":"Die Anforderung ist ungültig.","ModelState":{"model.Title.text":["Cannot deserialize the current JSON objec (truncated...) in .../vendor/guzzlehttp/guzzle/src/Exception/RequestException.php:113 Stack trace: #0 .../vendor/guzzlehttp/guzzle/src/Middleware.php(65): GuzzleHttp\Exception\RequestException::create(Object(GuzzleHttp\Psr7\Request), Object(GuzzleHttp\Psr7\Response)) #1 .../vendor/guzzlehttp/promises/src/Promise.php(204): GuzzleHttp\Middleware::GuzzleHttp{closure}(Object(GuzzleHttp\Psr7\Response)) #2
.../vendor/guzzlehttp/promises/src/Promise.php(153):
GuzzleHttp\Promise\Promise::callHandler(1, Object(GuzzleHttp\Psr7\Response), NULL) #3
.../ap in .../vendor/guzzlehttp/guzzle/src/Exception/RequestException.php on line 113

My addendum:

Thanks again for your help. I am working on the problem again and I keep getting the error 500 Internal Server Error.
Can you help me please?

Uncaught GuzzleHttp\Exception\ServerException: Server error: 'POST https://app.billbee.io/api/v1/products' resulted in a '500 Internal Server Error' response:

#75