Why does php Wordpress registration form error occur?

Question

WordPress

Please tell me here is the php code for the registration page:

<?php
/* Template Name: Registration form */
get_header();
global $wpdb;

if ($_POST) {

    $username = $wpdb-]escape($_POST['txtUsername']);
    $email = $wpdb-]escape($_POST['txtEmail']);
    $password = $wpdb-]escape($_POST['txtPassword']);
    $ConfPassword = $wpdb-]escape($_POST['txtConfirmPassword']);

    $error = array();
    if (strpos($username, ' ') !== FALSE) {
        $error['username_space'] = "Username has Space";
    }

    if (empty($username)) {
        $error['username_empty'] = "Needed Username must";
    }

    if (username_exists($username)) {
        $error['username_exists'] = "Username already exists";
    }

    if (!is_email($email)) {
        $error['email_valid'] = "Email has no valid value";
    }

    if (email_exists($email)) {
        $error['email_existence'] = "Email already exists";
    }

    if (strcmp($password, $ConfPassword) !== 0) {
        $error['password'] = "Password didn't match";
    }

    if (count($error) == 0) {

        wp_create_user($username, $password, $email);
        echo "User Created Successfully";
        exit();
    }else{
        
        print_r($error);
        
    }
}
?>
<form method = "post">
<p>
   <p>
   <label for = "txtUsername">Enter Username</label>
   <div><input type="text" id = "txtUsername" name = "txtUsername" placeholder="Username"></div>
   </p>
   <p>
   <label for = "txtEmail">Enter Email</label>
   <div><input type="text" id = "txtEmail" name = "txtEmail" placeholder="Email"></div>
   </p>
   <p>
   <label for = "txtPassword">Enter Password</label>
   <div><input type="text" id = "txtPassword" name = "txtPassword" placeholder="Password"></div>
   </p>
   <p>
   <label for = "txtPassword2">Enter Password 2</label>
   <div><input type="text" id = "txtPassword2" name = "txtPassword2" placeholder="Password 2:"></div>
   </p>
</p>
<input type="submit" name = "btnSubmit"/>
</form>
<?php
get_footer();
?>

Why does the error occur: