Guestbook installation

I found this template from https://geeksmakemehot.com/wordpress-guestbook/, and I tried installing the guestbook as directed but it came out to be all codes for me. Here’s my site: https://www.cheeseballonline.com. If you click on guestbook, you’ll see what came out of my attempt. ?? Can anyone help or tell me what I’m doing wrong?