New shortcode with corrections.

shortcode-fix.php

<?php
// Add Shortcode
function subform() {
    // Get the current user
    $current_user = wp_get_current_user();
    
    // Check if user is logged in and get the nickname
    if ($current_user->exists()) {
        $user_display_name = $current_user->nickname;
        // If nickname is empty, fall back to display_name
        if (empty($user_display_name)) {
            $user_display_name = $current_user->display_name;
        }
    } else {
        $user_display_name = 'Guest';
    }

    // Get the blog name
    $blog_name = get_bloginfo('name');

    // Build the form output
    $output = '<p>Hey ' . esc_html($user_display_name) . ', welcome to ' . esc_html($blog_name) . '! You can subscribe to our newsletter here:</p>';
    $output .= '<form action="/thank-you">';
    $output .= '<label for="email">Enter your email:</label>';
    $output .= '<input type="email" id="email" name="email">';
    $output .= '<input type="submit" value="Submit">';
    $output .= '</form>';

    // Return the output
    return $output;
}
add_shortcode('subscriptionform', 'subform');