It doesn't work if db table prefix is different than "wp_"

Inside function “file_during_checkout()” at /inc/functions.php you used:

$myposts = $wpdb->get_results("
	SELECT
	wp_posts.*
	FROM
	wp_posts
	WHERE
	wp_posts.post_parent = '".$wc[1]."'
        AND
	wp_posts.post_type = 'attachment'
	AND
	((wp_posts.post_status = 'inherit'))
        ORDER BY
	wp_posts.post_date
	DESC
	LIMIT 0, 1"
);

If someone use a different table prefix (ME), It will break.

So I suggest you to change from wp_posts to $wpdb->prefix as you can see at doc.

https://www.ads-software.com/plugins/easy-upload-files-during-checkout/