unity unitywebrequest 400 method not allowed error

Question

I am trying to send JSON to my python flask web server but when I run the code I got "method not allowed" for some reason

IEnumerator Upload()
{
    var something = new Data();
    //Debug.Log("something: " + something);

    var jsonString = JsonUtility.ToJson(something);
    //Debug.Log("jsonString: " + jsonString);

    using (var www = UnityWebRequest.Post("http://localhost:8000/test", jsonString))
    {
        www.SetRequestHeader("Content-Type", "application/json");

        yield return www.SendWebRequest();

        if (www.result != UnityWebRequest.Result.Success)
        {
            Debug.Log(www.error);
        }
        else
        {
            Debug.Log("Form upload complete!");
        }

    }
}

this is the server side:

@app.route('/test', methods = ['POST'])
def testpost():
    if request.method == "POST":

        data = request.get_json()

        print(data)
        return jsonify(data)