Undefined index $_POST // Uncaught SyntaxError: Unexpected token < in JSON at position 0

Question

Recently I've started to learn web dev in a Udemy course. When I use the code below, the error "Uncaught "SyntaxError: Unexpected token < in JSON at position 0" comes to the console. After looking at it, I realized that the problem was "Notice: Undefined index: accion" followed by the file name and line causing the notice. So, I asked my teaching assistant and he just told me to put "error_reporting(E_ALL ^ E_NOTICE);" in the code, like this:

<?php

**error_reporting(E_ALL ^ E_NOTICE);**

if($_POST['accion'] == 'crear') {
    // Creará un nuevo registro den la base de datos

    require_once('../funciones/bd.php');

I know that is to change the error reporting level and makes my code work but is not the solution I am looking for.

I've tried to use the function "isset" in different ways and it doesn´t work (I'm probably doing something wrong).

 1. if isset(($_POST['accion']) == 'crear') {
 2. if isset(($_POST['accion'] == 'crear')) {

I've also tried to set the variable at the beginning, like this:

$accion = $_POST['accion'];

But it doesn´t work either. I don't know what else to do. I've been Googling and I've seen similar things that has been fixed using the function "isset" when using "$_POST" but the only difference is that I use the equal operators (don't know if that has something to do). And as mentioned above, I've also tried to set this variable...

<?php

if($_POST['accion'] == 'crear') {
    // Creará un nuevo registro den la base de datos

    require_once('../funciones/bd.php');

    // Validar las entradas
    $nombre = filter_var($_POST['nombre'], FILTER_SANITIZE_STRING);
    $empresa = filter_var($_POST['empresa'], FILTER_SANITIZE_STRING);
    $telefono = filter_var($_POST['telefono'], FILTER_SANITIZE_STRING);

    try {
        $stmt = $conn->prepare("INSERT INTO contactos (nombre, empresa, telefono) VALUES(?, ?, ?)");
        $stmt->bind_param("sss", $nombre, $empresa, $telefono);
        $stmt->execute();
        if ($stmt->affected_rows == 1) {
            $respuesta = array (
                'respuesta' => 'correcto',
                'datos'=> array(
                    'nombre' => $nombre,
                    'empresa' => $empresa,
                    'telefono' => $telefono,
                    'id_insertado' => $stmt->insert_id
                )
            );
        }

        $stmt->close();
        $conn->close();

    } catch(Exception $e) {
        $respuesta = array(
            'error' => $e->getMessage()
        );
    }

    echo json_encode($respuesta);
}

if($_GET['accion'] == 'borrar') {
    require_once('../funciones/bd.php');

    $id = filter_var($_GET['id'], FILTER_SANITIZE_NUMBER_INT);

    try {
        $stmt = $conn->prepare("DELETE FROM contactos WHERE id = ? ");
        $stmt->bind_param("i", $id);
        $stmt->execute();
        if($stmt->affected_rows == 1) {
            $respuesta = array (
                'respuesta' => 'correcto'
            );
        }
        $stmt->close();
        $conn->close();
    } catch(Exception $e){
        $respuesta = array(
            'error' => $e->getMessage()
        );
    }
    echo json_encode($respuesta);
}
if($_POST['accion'] == 'editar') {
    //echo json_encode($_POST);

    require_once('../funciones/bd.php');

    // Validar las entradas
    $nombre = filter_var($_POST['nombre'], FILTER_SANITIZE_STRING);
    $empresa = filter_var($_POST['empresa'], FILTER_SANITIZE_STRING);
    $telefono = filter_var($_POST['telefono'], FILTER_SANITIZE_STRING);
    $id = filter_var($_POST['id'], FILTER_SANITIZE_NUMBER_INT);

    try {
        $stmt = $conn->prepare("UPDATE contactos SET nombre = ?, telefono = ?, empresa = ? WHERE id = ?");
        $stmt->bind_param("sssi", $nombre, $telefono, $empresa, $id);
        $stmt->execute();
        if($stmt->affected_rows == 1){
            $respuesta = array(
                'respuesta' => 'correcto'
            );
        } else {
            $respuesta = array(
                'respuesta' => 'error'
            );
        }
        $stmt->close();
        $conn->close();
    } catch(Exception $e){
        $respuesta = array(
            'error' => $e->getMessage()
        );
    }
    echo json_encode($respuesta);




}

?>

The notice comes in the line for $_POST['accion'] == 'crear' when trying to delete ($_GET['accion'] == 'borrar') and in the line for $_GET['accion'] == 'borrar' when trying to create ($_POST['accion'] == 'crear').