PHP SQL Why does this give me an error without details - php

Question

Hi I'm trying to insert data in my database. But I keep on getting the same error for example:
Error: INSERT INTO users (username, password) VALUES ('fff', '$2y$10$YUd1AErIj4RGRnjkFkYlkOn.s9OV62sq8.HVGO2jeE8dSthpgp6ey');
without any details which is very frustrating. I'm new to PHP and SQL so it's not the best written code ever and I know I should use prepared statements.
<?php
require_once '../connection/connection.php';
/**
* Created by PhpStorm.
* User: ezrab
* Date: 3/14/2018
* Time: 5:40 PM
*/
$username = $_POST['username'];
$password = $_POST['password'];
//var_dump($hashed_password);
if (isset($_POST['submit'])) {
if (!empty($username) || !empty($password)) {
if (preg_match('/^[A-Za-z]?[A-Za-z ]*$/', $username) || preg_match('/^[A-Za-z]?[A-Za-z ]*$/', $password)) {
$hashPwd = password_hash($password, PASSWORD_DEFAULT);
$sql = "INSERT INTO users (username, password) VALUES ('$username', '$hashPwd');";
if ($conn->query($sql) === TRUE) {
echo "Worked!";
} else {
echo "Error: " . $sql . "<br>" . $conn->error;
}
} else {
echo "You can't use certain characters.";
}
} else {
echo "You have to fill in all fields.";
}
} else {
echo "THOU SHALL NOT PASS!";
}
$conn->close();
EDIT: Added my connection.php file for more information.
<?php
$servername = "-----";
$username = "-----";
$password = "------";
$dbname = "------";
// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}
$conn->close();

Answer 1

EDIT:
Take
$conn->close();
out of connection.php and problem should be solved
You were opening and then immediately closing the connection before a query could be made