Prepopulate Field

Hi,
You can’t do that with our current settings.
But you can do it by custom js and php codes.

Here is the example,

add_action('wp_footer', 'footer_script');
function footer_script() {
    global $current_user;
    wp_get_current_user() ;
    $user_name = $current_user->display_name 
                    ? $current_user->display_name 
                    : $current_user->user_login;
    ?>
    <script>
        jQuery(document).ready(function () {
            // Replace #some_element_id with your field element id
            jQuery('#some_element_id').val('<?php echo $user_name; ?>');
        });
    </script>
    <?php
}

Add this code to your functions.php file in the theme root folder. Replace the #some_element_id with your element id.

Thank You!

Can you please share the error log
Or error that is displayed.