You're going to have to use a third party service like smartgb.com for your guestbook, I'm afraid. Neocities doesn't support server-side scripting like PHP.
afaik, sadgrl is able to have a php guestbook because she uses leprd.space. Here's her tutorial on setting up your own guestbook this way: https://learn.sadgrl.online/php-project-building-a-guestbook/ . Bear in mind, I've tried this method myself and was never able to get it off the ground (I don't think leprd.space is accepting new users at this time.)
hey hey I'm late to the party but what the others said is true! if you have any specific questions about building one in php/sql, feel free to shoot me an email (it's on my about page!)
oh wow!! thank you so much!! <3 your site is so inspirational. thanks for sharing so many tips and tricks for neocities.
Thank you so much!! <3 Your website must have come a long way because it looks amazing! Especially love your logo! :D
Thank you! <3 So happy when people like the pixel art header I made. And it's a bit crazy to think about but I've been on Neocities for nearly 6 months, and have been building and adding stuff to my site for that time. One of the best decisions I ever made, there are so many wholesome, creative, and kind people on Neocities. :3
Thank you <3 your site is awesome!