How can I fix the error "code 501, message Unsupported method ('POST')" with Python?

I made a web page in HTML, CSS and JS where there is a form that must store data in a SQLite database and I have used Python (with Flask) as the intermediary API between the form and the database. The problem is that when I register the data by clicking on the register button, in the console (because I am using Python's localhost) I get this message:code 501, message Unsupported method ('POST')"POST /subpaginas/database/machine_reg HTTP/1.1" 501 -

How can I solve this problem? Here my form codes, in javascript and the Python code

Form html code:

<div class = "reg_form"><form id = "registrationForm" onsubmit="submitForm(); return false;"><div class = "row"><div class="col-md-4"><label for="validationDefault01" class="form-label">No. serie</label><input type="text" class="form-control" id="validationDefault01" required></div><div class="col-md-4"><label for="validationDefaultUsername" class="form-label">Ubicación</label><div class="input-group"><span class="input-group-text" id="inputGroupPrepend2">📍</span><input type="text" class="form-control" id="validationDefaultUsername" aria-describedby="inputGroupPrepend2" required></div></div><div class="col-md-4"><label for="validationDefault04" class="form-label">Estado</label><select class="form-select" id="validationDefault04" required><option selected disabled value="">Elige...</option><option>Encendido</option><option>Apagado</option></select></div><p id="registrationMessage" style="display: none;">Se han registrado los datos.</p><div class="col-12"><button class="btn btn-primary" type="submit">Registrar</button></div></div></form>

JavaScript code:

function submitForm(){    var form = document.getElementById('registrationForm');    var formData = new FormData(form);    var xhr = new XMLHttpRequest();    xhr.open("POST", "http://localhost:8000/subpaginas/database/machine_reg");    xhr.setRequestHeader("Content-Type", "application/json");    xhr.onreadystatechange = function()    {        if (xhr.readyState == 4)        {            if (xhr.status == 200)            {                var registrationMessage = document.getElementById('registrationMessage');                registrationMessage.style.display = 'block';                setTimeout(function()                {                    registrationMessage.style.display = 'none';                }, 3000);            } else            {                console.error(xhr.statusText);            }        }    };    xhr.send(formData);   /* var jsonData = {};    formData.forEach(function (value, key) {        jsonData[key] = value;    });    xhr.send(JSON.stringify(jsonData));*/}

Python code:

import sqlite3from flask import Flask, request, jsonifyapp = Flask(__name__)@app.route('/subpaginas/database/machine_reg', methods = ['POST'])def registrar_maquina():    #data = request.json    #data.get    serial_no = request.form.get('validationDefault01')    status = request.form.get('validationDefault04')    location = request.form.get('validationDefaultUsername')    conn = sqlite3.connect('baseuno.sqlite')    cursor = conn.cursor()    cursor.execute(f"INSERT INTO machines (serial_no,status,location) VALUES ({serial_no}, {status}, {location})")    conn.commit()    conn.close()    return jsonify({"message" : "Maquina registrada exitosamente"})if __name__ == '__main__':    app.run(debug=True , port=8000)

Note: The HTML code where the form is located and the JS are located in the same folder, which is a subfolder since the forum is located on a subpage of my main page, the Python code and the database are located in a subfolder that is in the same folder as the form HTML

I tried using this block of code (found as a comment in the JavaScript code):

var jsonData = {};    formData.forEach(function (value, key) {        jsonData[key] = value;    });    xhr.send(JSON.stringify(jsonData));

And it didn't work either