Do not Access Superglobal $_POST Array Directly Apache Netbeans IDE 13

Question

I am trying to create a simple sign up function for a website. I get the error above when I click enter after filling out the sign up form. I believe the error refers to the following lines of code, specifically filtering on the $_POST function. Any help would be appreciated. See code below:

<?php 
 include("connection/conn.php");
 include("connection/showerrors.php");
 session_start();

 // initializing variables
 $StudentName = "";
 $StudentEmail = "";
 $errors = array(); 

 // REGISTER USER
 if (isset($_POST['reg_user'])) {
  // receive all input values from the form
 $StudentName = mysqli_real_escape_string ($conn, $_POST['StudentName']);
 $password_1 = mysqli_real_escape_string ($conn, $_POST['password_1']);
 $password_2 = mysqli_real_escape_string ($conn, $_POST['password_2']);
 $StudentEmail = mysqli_real_escape_string ($conn, $_POST['StudentEmail']);