POST Request works with Postman but not Guzzle 7

Can someone help me with this? I've researched this and tried a lot of examples on SO and other sites, still no success. What it's supposed to do is create an organization (client org) on the simpli.fi ad publishing server.

Today, I decided to test with Postman and got it to work. When using Guzzle, I get various errors but this is the one it lands on the most:

curl_setopt_array(): cannot represent a stream of type Output as a STDIO FILE*

Creating the Client:

 $client = new Client([
         'debug' => true,
         'base_uri' => self::$baseUrl,
         'headers' => [
                'csrf-token' => csrf_token(),
                'x-app-key'   => self::$appKey,
                'x-user-key'  => self::$userKey,
                'Content-Type' => 'application/json',
                'Accept' => 'application/json',
         ]
 ]);

JSON data:

 $data = json_encode([
            'organization' => [
                'name' => "Pete's Pet Pagoda",
                'parent_id' => self::$parentId,
            ]
 ]);

Create the POST Request

  $endpoint = '/organizations';

  $response = $client->request('POST', $endpoint, [
         'timeout' => 30,
         'json' => $data,
  ]);