1. The fix_cache_key.php file needs to be copied into the main (or "root" or "top" or whatever you wanna call it) folder where your Zen Cart files are located. That's where the main index.php file exists. You'll also find some other files there like ipn_main_handler.php, nddbc.html, etc. In your case I believe you called the folder "mystore".
2. The message seen in the picture you shared means that your database credentials (the DB_XXXXXX settings) in your configure.php files are incorrect, or that the MySQL service isn't running on your server (in this case "server" means your XAMPP PC).
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