Curl request doesn't login to my API, why?

Question

I've been working on this code to log in to my API. But every time I try to login using CURL I get the following error

{"message":"The request entity's media type 'multipart/form-data' is not supported for this resource.","exceptionMessage":"No MediaTypeFormatter is available to read an object of type 'Credentials' from content with media type 'multipart/form-data'.","exceptionType":"System.Net.Http.UnsupportedMediaTypeException","stackTrace":" at System.Net.Http.HttpContentExtensions.ReadAsAsync[T](HttpContent content, Type type, IEnumerable1 formatters, IFormatterLogger formatterLogger, CancellationToken cancellationToken)\r\n at System.Web.Http.ModelBinding.FormatterParameterBinding.ReadContentAsync(HttpRequestMessage request, Type type, IEnumerable1 formatters, IFormatterLogger formatterLogger, CancellationToken cancellationToken)"}

My code is simply meant to login to my ERP system using a static username and password. The code is as follows:

$logindata = array(
    'name' => 'username',
    'password' => 'password'
);
$headers = array(
    'Content-Type' => 'application/json'
);
$ch = curl_init($url);
//        curl_setopt($ch,CURLOPT_COOKIEJAR, $this->cookie_jar);
//        curl_setopt($ch,CURLOPT_COOKIESESSION, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, False);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, False);
curl_setopt($ch, CURLOPT_ENCODING, "");
curl_setopt($ch,CURLOPT_MAXREDIRS,0);
curl_setopt($ch,CURLOPT_TIMEOUT,30);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, $logindata);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
$response = curl_exec($ch);
$err = curl_error($ch);
if ($err) {
    echo "cURL Error #:" . $err;
} else {
    echo $response;
}

What am I doing wrong? and how can I fix it?

EDIT 1: I tried curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($logindata)); and while that stopped showing the error that I showed earlier, it still says that I'm not logged into the API and thus I'm still unable to use it further.

string(36) "{"message":"You are not logged in."}"