Google API 2.1 - Error [product] INSERT request must specify product

Question

first of all, my english is not that good. I hope you still understand what I mean.

I'm trying to upload a product to my feed using google api 2.1. Here, however, I always get an error message that I can't make any sense of.

Curl request to https://shoppingcontent.googleapis.com/content/v2.1/{merchantId}/products?feedId=XXX

Link to Doc https://developers.google.com/shopping-content/reference/rest/v2.1/products/insert

public function postRequest($url, $data, $accessToken)
    {
        $ch = curl_init();
        $headers = array(
            'Accept: application/json',
            'Content-Type: application/json',
            "Authorization: $accessToken"

        );
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
        curl_setopt($ch, CURLOPT_HEADER, 0);
        $body = json_encode($data);

        curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
        curl_setopt($ch, CURLOPT_POSTFIELDS, $body);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

        // Timeout in seconds
        curl_setopt($ch, CURLOPT_TIMEOUT, 30);

        $data = curl_exec($ch);
        $data = json_decode($data);
        return $data;
    }

 $productData = [
      "merchantId" => GOOGLE_MERCHANT_ID,
      "offerId" => "DE_" . $modelNumber,
      "title" => $productName,
      "description" => $description,
      "link" => $productLink,
      "imageLink" => $mainImageLink,
      "contentLanguage" => $this->countriesData["DE"]["langIso6391Codes"],
      "targetCountry" => $this->countriesData["DE"]["cldrTerritoryCode"],
      "channel" => "online",
      "adult" => false,
      "kind" => "content#product",
      "brand" => $manufacturerName,
      "googleProductCategory" => $googleCategory,
      "gtin" => $productEan,
      "mpn" => $modelNumber,
      "price" => [
        "value" => $productPrice,
        "currency" => $this->countriesData["DE"]["currency"]
      ],
      "productWeight" => [
        "value" => $productWeight,
        "unit" => "kg"
      ],
      "shipping" => [
        "price" => [
          "value" => $shippingPrice,
          "currency" => $this->countriesData["DE"]["currency"]
        ],
        "country" => $this->countriesData["DE"]["cldrTerritoryCode"],
        "region" => $this->countriesData["DE"]["region"],
        "service" => $shippingTimeName,
      ],
      "productTypes" => ["test"],
      "identifierExists" => true,
      "source" => "api",
      "availability" => "on stock",
      "condition" => $this->countriesData["DE"]["productCondition"],
    ];

Here I get the following error message:

object(stdClass)#32 (1) {
  ["error"]=>
  object(stdClass)#31 (4) {
    ["code"]=>
    int(400)
    ["message"]=>
    string(305) "Invalid JSON payload received. Unknown name "price": Cannot find field.
Invalid JSON payload received. Unknown name "productWeight": Cannot find field.
Invalid JSON payload received. Unknown name "shipping": Cannot find field.
Invalid JSON payload received. Unknown name "productTypes": Cannot find field."
    ["errors"]=>
    array(1) {
      [0]=>
      object(stdClass)#44 (2) {
        ["message"]=>
        string(305) "Invalid JSON payload received. Unknown name "price": Cannot find field.
Invalid JSON payload received. Unknown name "productWeight": Cannot find field.
Invalid JSON payload received. Unknown name "shipping": Cannot find field.
Invalid JSON payload received. Unknown name "productTypes": Cannot find field."
        ["reason"]=>
        string(7) "invalid"
      }
    }
    ["status"]=>
    string(16) "INVALID_ARGUMENT"
  }
}

If I comment out the criticized fields:

      // "price" => [
      //   "value" => $productPrice,
      //   "currency" => $this->countriesData["DE"]["currency"]
      // ],
      // "productWeight" => [
      //   "value" => $productWeight,
      //   "unit" => "kg"
      // ],
      // "shipping" => [
      //   "price" => [
      //     "value" => $shippingPrice,
      //     "currency" => $this->countriesData["DE"]["currency"]
      //   ],
      //   "country" => $this->countriesData["DE"]["cldrTerritoryCode"],
      //   "region" => $this->countriesData["DE"]["region"],
      //   "service" => $shippingTimeName,
      // ],
      // "productTypes" => ["test"],

I get the following error message

object(stdClass)#32 (1) {
  ["error"]=>
  object(stdClass)#31 (3) {
    ["code"]=>
    int(400)
    ["message"]=>
    string(45) "[product] INSERT request must specify product"
    ["errors"]=>
    array(1) {
      [0]=>
      object(stdClass)#44 (3) {
        ["message"]=>
        string(45) "[product] INSERT request must specify product"
        ["domain"]=>
        string(6) "global"
        ["reason"]=>
        string(8) "required"
      }
    }
  }
}

Do any of you have any idea where my mistake is?