Problem with registration system

When I try to sign in to the registration system I fill all the fields and I get this error: "User could not be added to the database!"
I don't know what I did wrong. I created a database named "users" on phpmyadmin and I created a table with the same name where I set up the id, the username, the password and the avatar.
Then I wrote this php script:

<?php
session_start();
$_SESSION['message'] = '';
$mysqli = new mysqli("localhost", "root", "mypass123", "users");
//the form has been submitted with post
if ($_SERVER["REQUEST_METHOD"] == "POST") {

//two passwords are equal to each other
if ($_POST['password'] == $_POST['confirmpassword']) {

//set all the post variables
$username = $mysqli->real_escape_string($_POST['username']);
$email = $mysqli->real_escape_string($_POST['email']);
$password = md5($_POST['password']); //md5 has password for security
$avatar_path = $mysqli->real_escape_string('images/'.$_FILES['avatar']['name']);

//make sure the file type is image
if (preg_match("!image!",$_FILES['avatar']['type'])) {

//copy image to images/ folder
if (copy($_FILES['avatar']['tmp_name'], $avatar_path)){

//set session variables
$_SESSION['username'] = $username;
$_SESSION['avatar'] = $avatar_path;
//insert user data into database
$sql = "INSERT INTO users (username, email, password, avatar) "
. "VALUES ('$username', '$email', '$password', '$avatar_path')";

//if the query is successsful, redirect to welcome.php page, done!
if ($mysqli->query($sql) === true){
$_SESSION['message'] = "Registration succesful! Added $username to the database!";
header("location: welcome.php");
}
else {
$_SESSION['message'] = 'User could not be added to the database!';
}
$mysqli->close();
}
else {
$_SESSION['message'] = 'File upload failed!';
}
}
else {
$_SESSION['message'] = 'Please only upload GIF, JPG or PNG images!';
}
}
else {
$_SESSION['message'] = 'Two passwords do not match!';
}
}
?>
<link href="//db.onlinewebfonts.com/c/a4e256ed67403c6ad5d43937ed48a77b?family=Core+Sans+N+W01+35+Light" rel="stylesheet" type="text/css"/>
<link rel="stylesheet" href="form.css" type="text/css">
<div class="body-content">
<div class="module">
<h1>Create an account</h1>
<form class="form" action="form.php" method="post" enctype="multipart/form-data" autocomplete="off">
<div class="alert alert-error"><?= $_SESSION['message'] ?></div>
<input type="text" placeholder="User Name" name="username" required />
<input type="email" placeholder="Email" name="email" required />
<input type="password" placeholder="Password" name="password" autocomplete="new-password" required />
<input type="password" placeholder="Confirm Password" name="confirmpassword" autocomplete="new-password" required />
<div class="avatar"><label>Select your avatar: </label><input type="file" name="avatar" accept="image/*" required /></div>
<input type="submit" value="Register" name="register" class="btn btn-block btn-primary" />
</form>
</div>
</div>



What did I do wrong?

 

Nevermind, I'm tired woo! I didn't create the email field on phpmyadmin and that's why I got the error. Everything works fine now! Thanks anyway.