Why am I getting a 404 error with my custom PHP app?

[Mervin_UWS]

Hi, I tried created a custom app using php and it was hosted to my test hosting/server

(here's the video guide that I follow, part 1 video: https://www.youtube.com/watch?v=tX1E8fuesSE&list=PL-k7XHuPgeIsv2Q6QtJR__h3kkpmqNpH0&index=3)

Also I tried calling external api and include the computation of external api inside the functions.php (inc/functions.php)

I just want someone that can check my work if the code in my test hosting/server is correctly coded I can give access to it and can give also access via FTP

Cuz when I try to install the custom app nothings happen I just got 404 error.

Here's the app url:
http://casinomervs.site/app/mervs/

Allowed redirection URL(s):
http://casinomervs.site/app/mervs/generate_token.php
http://casinomervs.site/app/mervs/install.php

then I want to use this as external API, tho I have the computation but I don't know where to include it in code:
http://spotprice.uwsdev.com/metals

Please see attached file for the source code of custom app.